Most signature quilts I make I try to have signed before they are quilted and then get the remaining signatures at the party -- after its' quilted.
Frankly, I've never seen anyone not be able to sign the quilt, even after it's quilted, and some of those have been washed A LOT and the signatures held up, did not run or show through, etc.
In fact, it seems so easy to sign after the quilting that I'm not sure it is really worth getting all those signatures ahead of time. Just wait for the party and get out the pens.
Of course, after the signing, all signatures must be heat-set with an iron before washing.
BethMI