Hey Sven! Sorry for the delayed answer! Welcome to the Stan forum!
I’m definitely not a brms expert, but I have asked a similar question a while ago. Not sure if it has been implemented yet, but I guess not since brms 3 is not out, @paul.buerkner ? If you are willing to work in Stan directly this is pretty easy to do – you could for example use brms::make_stancode and go from there.
Cheers,
Max