Thanks for your patient and methodical approach. I have used your suggestion but gone in perhaps a slightly different route (see later, below).
I think one of the issues is that when you add an instrument to a score, it is only the notational definition that is involved. For example, you don’t add a VSL BBO Jupiter Horn section or a Spitfire Horn section; you just add a horn section. And although there can be several possible horn playing configurations (e.g. Horn a4, horn a8 and horn a12), to Dorico the only distinction is between a single player and a section player: the Add Section Player doesn’t make the distinction between a4, a8, a12 nor the distiunction between VSL Horns a4 and Spitfire horns a4.
My desired workflow would be (after adding the three section horns into the score) to use a new dialog within Play → MST and Midi wherein I could scan possible end-points and select/assign the ones that suit each of my three horn definitions.
Anyhow, here is what I did to (partially) solve my problem; it is based on your suggestion:
a) I built a single, ridiculously large Dorico project containing all my likely-to-be-used VSL instrument definitions & maps, and saved it as a Project Template (VSL FULL Orchestra). It names the different horns a4, a8, a12, etc.
b) I also saved a global view of all the endpoints (Play → VST and Midi → Save Endpoint configuration (2nd icon at the bottom) in that project. I called it VSL FULL Orchestra and it is stored in AppData\Roaming\Steinberg\Dorico 6\EndpointConfigs.
c) I then added this single VSL FULL Orchestra endpoint IN FRONT of the SILENCE entry in a new Playback template which I called VSL Playback. Later, I will add that endpoint in front of the Vienna Assistant-generated playback template content.
d) I then set this VSL Playback template as my default playback template, in preferences.
Now for the Creation part:
a) When I start a new project, I will select the 'FULL VSL orchestral as my initial project creation template – from the Orchestral category under ‘Create New…’; Dorico will create the new Project with all my VSL instruments and their playback settings.
b) Of course, I will make sure NOT to activate the project, at this point
c) I will then go into Setup and delete all unwanted instruments for the current project in mind.
d) I will then save that pruned version of the score as my new, working Dorico project.
e) For added safeguard (not to lose the content of the large orchestral beast) I will make the VSL FULL Orchestra template a read-only file, using Windows File Manager. I can change that prior to any updating of the template.
I think it will work. However, I still deplore the fact that I cannot easily create different instruments with their playback settings from Setup → Add Section player (it doesn’t discriminate a4, a8, a12 neither does it discriminate horns rendered by VSL, by Spitfire, by XYZ). I deplore having to go through that complicated (initial large project creation) and potentially risky (subsequent items deletion from the large project) process every time I create a new Dorico Project.