I think I’m wandering around the solution, but I don’t want to get ahead of myself.
Dorico stops auto-generating unmeasured tremolo as soon as the “tremolo” playing technique has been used in a user-created expression map (and set at the specified number of slashes in preferences). This means “tremolo” then becomes just another element in a string of criteria a user has to set in order for Dorico to find the path to the required sample. These strings are any combination of graphical symbols from the score that in this or that unique combo match to entry(s) in the expression map that triggers a sample.
A metaphor: Press the button when you see a “white”, “long”, “dress” (green long dress will be ignored). Or: play this sample when you see a combination of: staccato and mf. Or, mute, marcato and fff
In my case, that string for single hits was:
- playing technique (beater type) + note length (left untouched, so responding to the score)
The string for recorded tremolos had to be unique from the first one, therefore:
- playing technique (beater) + playing technique (tremolo) + note length (left unchanged)
Both of them worked like a charm. But it didn’t work when I simply added “crescendo” into a third string to trigger recorded crescendo rolls instead of the plain ones. I think that’s because Dorico was trying to add crescendo via CC1 (as it normally should - I could see it in the MIDI CC lane) while the rest of the string was identical to the previous one. In other words, “crescendo” was the wrong differentiator because it’s activated differently. At least that’s my assumption right now, but I can see it is available as a playback technique if I want it…
The solution, I believe is to once again ensure that I use a unique string used to trigger recorded crescendo rolls . Since I don’t want to touch “crescendo” for the time being, I will consider creating a different “length” string, which might make sense later on for the recorded measured tremolos (I have to map 26 articulations just for timpani) 
Hopefully, it will automatically get added to its own exclusion group in the expression map.
If I understand this topic correctly, Dorico expression map is based on Cubase but is slightly different and in some ways expanded. Cubase puts a limit of 4 elements in these strings (so called “combined techniques” in Dorico) and I believe in Dorico this number is unlimited and linked a lot more transparently to playing techniques. Conceptually, Dorico’s map is taking it to another level because the number of possible combinations is expanding astronomically.