It's annoying enough that the dropdown selections and other input is discarded as OOUI finishes doing its thing.

This is not true. Input made in OOUI widgets generated by PHP code (dropdowns, text inputs, checkbox, and so on) is not discarded when OOUI loads and these are turned into JS widgets. I just tested and this definitely still works as expected for me, on Special:Block as well as everywhere else. If you are really experiencing problems with this, please provide more details.