Call ahead to confirm, but perhaps a day pass to the AA Admirals Club will fit the bill. Their Boston site has showers and is located in Terminal B ...a lengthy walk from the VX gate, but a walk.. It opens at 4:45am.. I have one of the AAdvantage credit cards (that came with a bunch of points and a hefty annual fee) that includes access to the Admirals Clubs..But I think a day pass can be had for $50-$75..