Do you have plans on making Layouts saveable?
I always make my arrangements in a sequencer, and export this as a midifile into the scoring program.
If I could load a (user-defined) layout-preset on this imported midi file, I would be almost finished in two clicks…
(BTW; I begged Finale to implement this "Library) feature since about 12 years… no reaction).
I would be very happy to see this happen. 
Why not construct a template yourself and import your MIDI into it?
Hmm. I didn’t really check that yet.
(Is it that easy?)
Oh. Maybe I should define “Layout Libraries” somewhat better:
I’m talking about a GLOBAL Library that I can “whip” over a “finished” score.
Oh, so that’s what you mean. No, there’s no way to do that as of yet. Either way, you should follow pianoleo’s advice for the time being. Because of Dorico’s understanding of Players and Layouts, you can save a blank document with no Players but with all your Layout and Engraving Options intact. Just open that document, Save As, and start adding players into which you can copy music into.
I recognise that you’re basically asking for an equivalent of Sibelius’s House Style function - I only meant to give a workaround for now.
I believe similar requests have been made in the past and something along these lines is already on the developers’ to-do list.
Great! - I’ll just wait, then…