I wore a mermaid dress for my wedding and although I would have been able to shimmy it up to pee, because I wore the sucky-in shorts that went up to just under my bust it was too awkward to try and pull the shorts down from out of the dress and then pull them all the way back up again. So i had to take my whole dress off to pee
I never planned on wearing the shorts until the week of my wedding (every
pair of undies showed a line - due to the style of dress it just hugged everywhere) and didnt think I would be the bride in the toilets with BMs holding her dress up while she peed!
Thankfully I didnt realise this until the actual day, otherwise I would have been stressing about it needlessly. I didnt need to go until we got to the reception where we had 1/2 hour set aside to chill/freshen up anyways. My BM had to hold up my dress for me while I peed, given that the whole thing was basically undone around me. And I didnt need to go again all night.
So its not such a big deal if you have to take your dress off - if your dress is that tight you probably wont need to go more than once. And your BMs/MOH will be close friends or family so they should be able to put up with it for your special day. Dont stress hun - I laugh thinking back to it!