Another option is a bag made for pack security in hostels. When I go to court, I have a suit under the armor, and, happily, covered and relatively secure parking. The jacket, gloves, and pants go in the bag, which is made of steel cable, which gets locked to the bike. I would never do this for street parking unless I really had to, but in parking garages or under a bike cover it does pretty well.
What I have is essentially this
, and it does actually fit under the seat, though I don't carry it there.
The easiest option, IMHO, is to lock a jacket and maybe pants using the orange cable on a decent disc-lock doubled back, though again, kinda asking for things to get fucked with that way. I think if I was regularly going to places with only street parking, particularly in the city, I'd get more locking, on-bike storage. I'm a little spoiled by using garages that are literally across the street from court houses, and thus cops.