Ummmm...Martha Stewart has done a candy "bar" on her wedding website before and if Martha's done it...it's CLASSY! lol!
Seriously though. It's a small thing that will be lots of fun for guests. I had really wanted to do this at my reception but I just didn't get it together in time. And since it's YOUR WEDDING, YOU'RE PAYING FOR IT and YOU really want it. . .you should just do it. I realize it's her house...but if it's mess she's concerned about - then it's a no go on the coffee 'cause that sh*t STAINS!
I say esp since you've alreayd bought the stuff, put your foot down and tell her you can't be talked out of it, it's part of your dream wedding fantasy ....
Of course be nice about it, but make your point. And make sure your FI will back you up too! :-)