I'm not sure if this is an ettiquette question, but here it goes.
I'm having a destination wedding in May 2014. There are 110 people on the guest list, but I have no clue who will actually be in attendance. Most ceremony/reception details are already taken care of by our planner, but the rehersal dinner is on us to figure out. FI and I had our hearts set on something fun and different so we were going to book a pirate themed dinner cruise for the wedding party, their dates, and our immediate families, about 28 people in total. The boat fits 60 and will only take private reservations at a minimum of 40. Paying for an extra 12 people puts it out of our budget. My mom asked if the boat had been booked and I told her that we haven't decided what we are going to do. Either just pay for our 28 guests, and not have it be a private event, or hold it somewhere else. FI and I were leaning towards non-private event. My mother hates the idea of sharing our RD with strangers. She'd rather I rent out the entire boat and then open up the extra spots and have guests who aren't in the wedding party pay me to go. I told her under no uncertain circumstances was I going to ask wedding guests for money or to pay me for anything.
If my mom offers to pay for the rehersal dinner so she can book the entire boat as a private party, and we accept, would it be inappropriate for me to tell her she can't fill the extra spots? Could I let her deal with the extra spots as she wishes? I'm concerned about going to later way as I don't want it to seem like I was playing favorites among guests with the extra invites to the RD. Plus, what if only 60 people come to the wedding? That could be leaving out only a few couples.
"There is always some madness in love. But there is also always some reason in madness."
-Friedrich Nietzsche, "On Reading and Writing"