Another bride on here asked this question a while ago and someone mentioned that for wine/champagne you can bring your own but there is a corking fee and I think she said it was $20+, which is a lot. You may be better off telling your planner you want to buy the champagne by the bottle. The cheaper one (not the fairy tale cuvee) is about $45 a bottle after taxes. If you serve half glasses you can get at least 6-8 glasses out of one bottle. That will save you $$ on the mimosas.
Edited to add: they will also charge you a bartender fee on top of the corking fee so it may not be cheaper in the end to bring your own.