I don't know of anything. U could possibly read some agriculture readings, info and forums on farmers growing sugar beets (most common brassica with most data- similar enough to compare to ur brassicas) (beets- which, yes, now have round up as option too). I'll bet if u Google "herbicides for sedge in sugar beets" or something like that- possibly find something. But - reality..... If it were mine....
Depending on how bad, how many acres, here's viable things I've done or options...
-carefully spray with roundup with back pack sprayer on a course stream where u limit spray that kills brassicas. I'll literally drop some radish seed where i spot spray too if open spots.
-pull by hand
-if it's overwhelming and brassicas are not going to over take it... I'll nuke the whole thing and drill in a mix.
I had one issue this year with a "new area" that hadn't been rotated in past... Weed seed bank was nuts. I had to let it rain and germinate a couple times and burn down twice. Then I finally put my seed down and doing great.
Good luck - hope turns out alright.