KevinP, I will tell you how but only only in return for one of your mystery bottles ;-)
I like your nomenclature of "source" and "target" stores, so I'll use that here...
I've done this a couple times, and each was different. Once I called the source store, they located the items, took the store # of my desired target, rang me up including UPS shipping cost and took my credit card # over the phone. I got a call from the target store a couple days later and picked up my order without issues. Another time a source store wouldn't transfer anything unless they were contacted by the target store. And one time the target store wouldn't initiate the transfer for me over the phone, they needed me to come in and present my credit card # in person.
Lastly, one time the target store did an inter-store transfer from the source store at no cost and held it for me when it arrived...this took a bit longer but still worked. I haven't tried recently but understand that there is a new $2/bottle charge for this service:
http://www.pawinetalk.com/talk/7_232_0.html
I think to a certain extent your success and/or the amount of hassle you encounter may depend on your relationship -- if any -- with the luxury wine buyer in the target store.
Good luck and let us know how it goes.