Chris, you should be just fine waiting until tomorrow. If this does turn out to be an abscess, they are usually slow growing and encapsulated in one area, plus Bandy isn't showing any signs of pain or lack of appetite. So I don't think waiting one day should be a problem as long as there aren't any changes in the lump or with Bandy.
Though the bacteria itself can be contagious, it really isn't a problem for your other buns. Most rabbits already are carriers of the bacteria and are just unaffected by it. It's a bit like if we had an infected wound. We all usually have the bacteria naturally on our skin anyways, and even though the bacteria itself could affect others, it's just not likely. Plus if it is an abscess, it's still encapsulated at this point. Usually when they are talking about the bacteria being contagious, they are talking about when a rabbit has a URI and is sneezing. It really doesn't seem like something you need to worry about at all.
I know you are worried about Bandy. Just remember that it may not even be an abscess. It's still possible that it's a cyst. But even if it is, though abscesses can be serious and some difficult to treat, they actually are a pretty common thing for rabbits to get, and most are treated successfully and without any problems. In fact just recently another RO member had a similar problem. Her bun ended up having a facial abscess. One vet told her it could be dental related. She got a second opinion and found that it was just a superficial abscess probably due to a bite. The wound healed up just fine without any issues. It's good to be aware of the possibilities and what needs to be done, just so you can make sure your bun gets the proper treatment, but you also don't want to jump too far ahead and presume the worst when it may end up being something minor and easily resolved. Bandy is still acting normally and being his usual fluffy self, which are all good indicators and signs.