I can save in Play mode the endpoint configuration of a midi instrument (16 channels, expression maps).
Is it possible to open a saved endpoint configuration of one particular midi instrument?
Unfortunately when I opened today my orchestra piece, one midi instrument (the one for the woodwinds) was empty, that is to say: the midi instrument was there, but there was no LoopMidi port in. So I had to restore it: selecting the right LoopMidi port, make sixteen channels and fill in the expression maps again. Then it worked okay. I realized, when I save the endpoint configuration and can open it later on, it cost less time to restore the configuration.
I apologize if I misunderstood your question - but I think the answer you are looking for is that Dorico is meant to save and restore Playback templates to a piece… Yes you can save the end points, but there is no direct way to use the saved end points except through a playback template.
A) Save the endpoints
B) Create a play back template
C) Add your saved endpoints to the playback template and Save the template
D) Restore the template whenever you like to any project.
I have sometimes fussed to myself at not being able to just open a saved endpoint in play mode , but I do understand the usefulness of the hierarchy when I make improvements to an instrument’s endpoint which automatically updates my brass template, which automatically updates my orchestra template and any custom templates based on that. Especially in your case, to add the extermal midi endpoint to a lot of different templates where you’d probably want it.
Thanks for your answer. I experimented a little with making a play back template. For me there is a problem. For for instance the strings I use several ports (midi instruments) for a section (has to do with avoiding overloading the processor). But the template give each section just one port. So I will have a look how I could change that. I think for this point I can find more clever solutions. But I want to compose also, and this process needs much time (for me it also a sport to figure it out 
).
Did you limit the number of Midi channels for each port to those actually in use?
Also, Dorico will automatically number identical players/instruments, like Violin, in the Score, but the Endpoint system will not take this numbering into account, so all players named Violin (section) will be treated identically when the Playback Template is loaded. This looks like a severe limitation… (I’ve yet to exploit possible workarounds though)
This is outside what I’ve delved into. But If you create an external MIDI instrument in Dorico and then click on the cog button, You will notice that the instrument can itself have many ports as well as channels. You could have say legato on one port and trills on another.
But if you want a single part playing the same articulation to go to more than one at the same time, then yah that is currently a limitation. You could though have horns 1,2,3 and 4 all going to different ports if you wanted.
I took the Strings template (section violins I, II, vla, vcl and bass) and set Violins I to Midi Bank 1 ch.1, Violins II to Midi Bank 2 ch.1, and the rest to Midi Bank 3, chs 1,2 and 3.
I then saved the endpoints, and applied them to a new Playback template.
When the template is activated Dorico removes the Midi Bank 2 and sets VIOLIN II to the same bank/channel as VIOLIN I
This happens even if Violin II has a unique expression map and midi channel…
EDIT:
tried the same procedure with VST instruments instead of MIDI instruments. There is still a problem: Dorico will not remove the Violin II VST Slot, but it will replace
the dedicated Violin II Expression map with that of Violin I, as soon as the Playback Template is applied…