Just made a couple of changes to the multi-signature GUI... essentially sanity testing to make sure that any pubkeys that you enter as part of the multisig transactions are valid addresses. Next I will be integrating custom M-of-N multisig transactions (upper limit: 15 of 20) into the GUI. From what I understand, 15 of 20 is the scripting constraint... feel free to correct me if I'm wrong.
After that, I'll finally add into the GUI the ability to create transactions offline, signing and broadcasting at the click of a few buttons.
I'm not sure 15 of 20 is the limit, but I'd test this out to see what you're actually able to submit.