Indeed. The fact that you cannot easily create a Layout that merges both Full Score and Parts, it is probably due to Dorico usage of specific Page Templates Sets and Layout Options, etc…
(but maybe from the print panel will, in future, be possible to merge them at the moment of printing/exporting…)
Anyway as a pure exercise (was fun), I made a Custom score Layout example, where the full score and the parts coexist (using custom page templates with strategical addressed Music Frames chains and Players assignments, and some inserted Page Templates Changes, etc…).
This is of course just a mere exercise in Dorico Music Frames chains functionality, as the workflow is quite convoluted, and error prone (as it uses “virtual” First and Default Page Templates for each player, as custom Page Templates), and has several other disadvantages (not separate Layout Options for example). I didn’t bother to put page numbers tokens, or headers tokens on the custom templates…
For whom may be interested, I attach the Dorico file:
unfied layout with full score and parts.dorico (685.8 KB)