I remember deadheading from DFW to LGA a few years ago. I was sitting next to one of those upper mgt types. We got chatting about the Shuttle. He told me that AA did not need to purchase a Shuttle from anyone because; With the aquisition of TWA, AA aquired enough slots and gates at LGA/BOS/DCA to start it's own shuttle. If you look at the Eagle schedule you can see that they are operating a shuttle but with very little publicity.
IMHO, when/if USAir sells the Shuttle that's when AA will market it's own. Competiting against an established Shuttle is hard to do but starting the AA Shuttle when someone else starts their new version of the USAir Shuttle (Shuttle by CAL/NWA/B6) makes more sence. It leaves plenty of market share up for grabs.
IMHO, the BOS facility is what AA would go after and from what I have read it's not for sale. Carty actually called about the possibility of purchaseing it a few years back.