They say they are not refundable. This has been a complaint on here, what if there is a tropical storm....you could go to guest relations and try to get a refund. If they saw you went to the park anyway and just didn't use it, well I doubt you could get your money back. So in your case, if you show up to the party and try it and it doesn't work and then you go to GR and I guess say you didn't get to the park early enough(because they will see you made it for MK)... if I was a CM I would know exactly what you are trying to do and tell you tough luck, but I'm not a CM.
My DD is a CM, she has different media to get into the parks, if the system messes up and tries the wrong ticket, she can't get in(reservation) until she goes to guest relations and gets them to switch it. I'm pretty sure it has been proven that with
Genie+ and others if you don't enter the park your MB/ticket won't work on the rides. Did people have success with Genie+ and entering just for a party? If it worked for that, then it might work. This seems like a lot of hassle to ride maybe 2 rides with the LL.