Thanks for sharing the minutes!
My 5 cents is that Stan is already flexible enough to let you code any necessary constraints - and nesting constraints in the order “built-in first, custom second” is also possible (if I understand the meaning of “nested constraints” correctly). So allowing “custom first, built-in second” - while definitely helpful in some cases - won’t IMHO bring a very big benefit. IMHO the only thing that’s not possible in the current version is to have a custom transform not apply the Jacobian correction when running optimizing (as you’ve already proposed some time ago: e.g., here and here and as I discussed here). So just wanted to +1 the idea while you are discussing constraints. It’s mildly related to some work I try to do with SBC for approximate algorithms, but it is not particularly blocking me on any specific project (and when truly needed, one can pass a boolean is_optimizing as data, it’s just not very elegant).
Thanks once again for all the work going into the language!