the whole point of not taking the whole unit off of the guitar is that it's a big hassle and unnecessary. I put one of the other screws in a baggy and took it with me to a couple of hardware stores.
As described by an employee at one of the stores, it is a chrome, truss-head philips 4mm threaded machine bolt, approximately 6mm long.
I still haven't heard from Allparts. I wish there was a way to order directly from Fender...
I'm hoping that I can find a 4mm bolt that is just longer and can be cut down to size.