I really would like, that the positioning of system text could be a bit more flexible without any workarounds.
If I’m in a modern arrangement, I don’t use rehearsal marks to name sections, but I name those with expressions like: Verse, Chorus, Refrain, Intro, Bridge, Überleitung and so on. If a name like this happens to be at a system start, I want it to be positioned at the start of the system (as it is possible with the rehearsal marks) and not being postioned according to the barline or note. In Sibelius there was an option to place text horizontally depending on the page and not on the note or barline (left-, right-handside, centered). Besides this positioning could sometimes also be useful with normal text. Actually I experimented with rehearsal marks to get those texts working, but the numbering made it unpossible and I still would like to use rehearsal marks to mark sections within those used expressions.
but the numbering made it unpossible
Given the number of advices from very competent people on this forum, I would think this is wrong. Numbering does not make it impossible, since you have a full control over the index value in the Properties panel. It might be that you do not like it or find it cumbersome, but it’s certainly not impossible — or I misunderstood your post 
This was recently discussed on the forum in this thread. I agree with the principle that there should be a dedicated feature for labeling sections within a flow, so we’ll think about how best to implement this in future.
Actually I read the mentioned posts and the advices from the experienced users before I made my post, but they’re for such simple tasks like ‘section labeling’ either too complicated or just workarounds with tools that are designed for other tasks.
And I would like to have the rehearsal marks and the section labeling both together, if I’m free to say so and then it’s in my opinion almost impossible to work with the index value in the properties panel, like you suggested Marc, or if so it takes hours.
So I go with Daniels statement and hope there will be a useful solution for section labeling in the near future.
Besides the question was not only: how to make section labeling, but it was also about positioning system text according to the page borders.
Dear Dorico-Team
I was really baffled 
by all the new features and – unfortunately – have to replace my old “mac pro” now, for I can’t update to the needed Mac 10.12. anymore. 
But I will gladly do that for the big benefit Dorico3 is offering. Me as a rather modern engraver, I am now really fine with Dorico’s functionality and I’m finally quiting Sibelius. But, as Steve Jobs would say: There’s one more thing (which is missing) … as i stated above in this topic I need:
boxed expressions like: Verse, Chorus, Refrain, Intro, Bridge, Überleitung and so on.
That means system Text that has the same functionality like markers and – on top of that – is placed on the left side of the page, if a system starts with something like “chorus” and even – in this case – suppresses the bar number. Right now I make this with normal system text, but I have to position each and every item manually in each part and by pity “propagate” does not help me there.
But you did an awesome job with the update 
+1 for a dedicated feature for labeling sections. Currently grappling with how to get a rehearsal mark, section name (system text), and player instructions (staff text) to not collide and/or do bad things to staff spacing. If I understand correctly, my best shot for reducing the amount I will need to tweak each part is to attach potentially colliding text items to different rhythmic positions, and then (?) maybe set some justification options.
+1 for this
+1
boxed expressions like: Verse, Chorus, Refrain, Intro, Bridge, Überleitung and so on.
Also +1 
But you did an awesome job with the update >