I have no idea how to accomplish the following, help is greatly appreciated.
Until now my use of the Pages/Master pages panels in Engrave mode has been basic. My needs have been not much out of the ordinary and I have become acquainted with the concept.
At the moment I am working on a score for symphony orchestra. On some pages, I’ve had to reduce the staff size to 0.8 mm to fit all the staves onto the page, there are multiple divisi in all strings and a lot of information. My publisher’s size options stop at A3 (they can print, but not bind, SRA3). Much of it is hardly legible.
For the densest pages, I’m considering adding an appendix after the last bar, with separate pages for the winds, brass and percussion, and the strings on other pages, for study and preparaiton purposes for the conductor.
My skills in the Pages world stops here. Can I copy music, create frames, make the necessary selection of staves, to add this appendix?
It sounds more like you are going to add new layouts in the appendix…
Create a wind/brass/strings layout in Setup mode (right panel, new part), in which you tick the appropriate flows and instruments. Rinse and repeat for strings or whatever you need. Once this is done and you have finished the Engraving, you might want to insert some page number changes so that these new layouts fit perfectly after the full score in the order you want. This is how I would proceed.
Yes and no. In 3.5 you can use Manual Staff Visibility to hide or show individual staves at any system break, regardless of whether those staves contain anything, so that bit’s easy.
What you can’t do in a Layout Frame (which is the type of music frame you’d need) is tell Dorico to start a frame at a specific bar: they always start at bar 1. You’d need to duplicate the relevant bars into a separate flow, I think.
Marc’s suggestion would work nicely, too 
Thanks both for your solutions, this sounds promising. I’ll wipe my tears for now, try and wrap my head around this and give it a serious try.
Just checking whether you’ve already used condensing to reduce the number of staves and allow for a slightly larger staff size? or is condensing really not possible due to the difference in material in this case?
Sadly there are no adjacent voices that are identical, so condensing is not an option.
They don’t have to be identical to share a staff - see here for some different condensing possibilities.
Absolutely – I was being imprecise: each staff contains music wildly different from the others, with great tone intervals up and down crossing each other and whatnot.
Could I ask a thing on a side path, something I have always wondered, often needed, and never found the answer to: Can one condense single pages? Or do one have to enable condensing, and turn off condensing manually for all pages save that single page you want to condense for whatever reason?
I’m fairly sure it’s the latter, that you’d need to enable condensing and then manage it (i.e. using condensing changes to control where condensing occurs).