Ah, I remember RabenLied (the author of the REP mod) tried this before, and he ran into some issues, too. Not sure whether he solved them. I'll ask him.

Yes, he confirmed that.
The main issue is the stim behavior, which includes movement - and that's why it does not work with structures. However, it is the only coded behavior that targets friendly, unstimmed entities (regardless of whether they are at max. HP).
There are only some workarounds for this, but they are far from perfect. For example making the structure apply stims in an aoe around it.

I'll see whether the code changes required to make normal stim behavior on structures work are somewhat easy (I hope so), then maybe I can make this possible in the next patch.

I made some progress and now somehow works. But the main problem now is that will only activate when enemy actors are close. Even though in Scope states TargetEnemy = False.

https://ibb.co/1Qg0LFm

I like the musicians as a static buff! :)
Anyway, I'm afraid this is as good as it gets with the current code. TargetEnemy in the Scope only makes sure the aoe buff does not affect enemy units. When it shoots is determined by the behavior that is framing the "weapon" in the respective trait file. A weapon without defined behavior will just use standard attack behavior (when enemies are in range). You'll see that healing and Wolf buff have a special behavior defined, so they are only triggered when friendly unbuffed/damaged units are in range. However, that does not work well with structures, currently.
I'll try to change this and make stim/heal from structures possible, even without enemies nearby.

Hmm, after writing this, I noticed there might be a workaround... you could probably make the weapon activate regularly, like an Owl spawns Mice regardless of whether enemies are nearby. There should be a variable like "RequiresTarget"... just look at the Owl's "weapon" and try to copy that!

That was a good thinking, but unfortunately is not working for me :(
Edit: Now it's working pretty fine! You are a hero. Now the only problem is that the structure is giving the buff even before the structure is build. But other than that, it's working!

Ahh, yes, that sounds familiar from RabenLied's experiments, too.
Well, guess that's as good as it gets for now. Really need some code changes in order to make it better.
(Unless you consider removing the build time for these guys a fix. )