Addition: it would not be analogous to the note values, maybe that’s what you meant by proportional?
Like the proportions reflecting the note values?
Dan, why would contemporary notation want to be restricted by boring rules like “two quarter notes equal one half note?” That is sooo 20th-century! 
Using 2 is only an approximation, because accidentals, clef changes, bar lines, etc can mess up an exact “mathematical” definition of proportional spacing
Dorico will “go down the note chain” as far as you like, so long as you have small enough note heads and don’t mind having no space between them. The standard size note head is 1.2 spaces wide, so Dorico won’t put notes closer than 1.2 spaces together even of you set the minimum space between notes to zero.
I mean, note spacing in contemporary music doesn’t need to be analogous.
If your piece doesn’t require a lot of small values, I think it is reasonable to use something less than two, but still reflect the relationships. In the end, the eyes sees proportions still.
Interesting, I should try out values like 16 once and see how it deals with that then 
I thought Dorico would just assume that anything under a 16th will probably be spaced as close as possible anyhow
You can’t enter values bigger than 2. (Boring!!!)
But you can use proportional spacing with very short notes if you want to.
Is it possible to have this sort of proportional spacing only for some parts of the music? I could certainly use it at some points in the piece, but I definitely can’t work with it for the entire piece.
One other advantage of tied notes being a single entity, which I use all the time, is the ability to lengthen or shorten notes by the rhythmic grid using keyboard shortcuts (alt-shift-arrow, either forwards or backwards). And if you have an unmetered bar where you need to add or subtract some beats, you can use the popover shift-B, and then “+1 q” for example. When I’m writing unmetered music these two things are incredibly useful.
If you really want to decide where the ties should go, then you may want to leave Force Duration on for the whole piece.
For double-clicking on a long note to add a playing technique, I’m not encountering the problem Johnabram describes above - it works fine for me, for some reason.
Is it possible to have this sort of proportional spacing only for some parts of the music? I could certainly use it at some points in the piece, but I definitely can’t work with it for the entire piece.
This I’m not sure about. Maybe multiple flows, where a new flow can start on the same page? Or use a hidden time signature?
Yes. Use Note spacing changes (Engrave menu) to change and reset note spacings!
Does this involve manually tweaking the proportionally notated section? Or do you mean I can reset it or make it proportional locally?
Agreed. That’s why I really hope that there will be an option to have force duration on by default. I would just turn it on and forget about it.
I anticipate that I will want it on for all of the music that I write in the future. For me, only bad things come from Dorico forcing on its own idea of how notes should be grouped.
You don’t need to tweak manually. Insert a note spacing change where you need a change, and another (set to reset) to cancel the change when you want your score to revert to the previous behavior. This means you can use it for one bar without needing to resort to manual note spacing !
Thanks for clearing that up!
It would be great to have a possibility to select a part of tied notes in Write mode. Currently, it is often inconvenient to work.
For example, ‘Shift + click’ could be used to select a specific notehead of tied notes in Write mode.
The note spacing change is what rob was showing in his screenshot.
Just set the spacing globally as you want it mostly, and then set a note spacing change locally where you want it, later set another one to reset it.
You could only do that by changing what shift+click already does (which is much more useful IMO).
It doesn’t happen all the time. When I wrote that, it did seem to be happening most of the time, but subsequent tries worked as expected. It feels like I’m clicking slightly off-grid or maybe my mouse is giving only a single click? I’ll let you know if I figure it out.
Indeed. Tantacrul made a great video on youtube going through many of the problems in the way that Dorico treats tied notes. Many Dorico users did not like it, but he certainly made many good points.
Oh never mind tantacrul, John Barron just shared a review on facebook that complains about the same problem.
Tantacrul has got his own notation program to play with now.
Which is sad in a way, because Musescore was a useful little app until somebody got the idea of taking over the entire universe in the next version.
EDIT: Since originally posting, I found the link posted below (I had searched before posting, but clearly not thoroughly enough!).
Hey there, I’m quoting you here as I could use the advice of a pianist user. I’m new to Dorico, and my current project is an adaptation of a violin/piano sonata for cello and piano. As I work on entering the piano score into Dorico, I am having particular trouble with slurs/ties between up-stem and down-stem notes on one stave. For example, I have an 1/8th note at the end of a bar that needs to simultaneously tie to an inner voice of the following bar, and also serve as the beginning of a long phrase slur which terminates on an up-stem dotted-half note a few bars later. The way the “voice” system works to determine up-stem and down-stem, it seems that I cannot have it both ways with the ties/slurs coming off of this note. I hope my description makes sense. Is there a way to deactivate the controls around slurs and apply them manually from one note to another note, without needing to be grouped into the same “voice?” I can see how the voicing system would be helpful for melodic instruments, but it seems in keyboard writing there needs to be flexibility on the way different voices are stemmed, for optimal readability. Thanks!