It would be possible to establish bgp with the HSRP vIP. However, I woudl refrain in implementing this in a live network.
BGP is TCP based and is very fickle in that sense. Things can get pear shaped if the TCP isn't established with the stanby router. Of course if you clear the bgp session manually that would work. What I am hinting at is that when one router ungracefully closes the session and the other router(s) don't know to reestablish the connection it can get ugly.
BGP is a beautifiul protocol much flexible than HSRP just because you want to save a /30 isnt much of a use. I would definelty prefer using 2 /30's and use attributes for manipulation in dual homed scenarios. Works well and scalable as well.
BTW is it mandatory to use bgp with HSRP in your network?