It won't fit under the seat but, the 12V compressor sold on this tool site:
http://buzzardgulch.com/BuzzardGulch/CatalogV2.htm really kicks ass (the one pictured on the home page is different in appearance to the actual compressor sold on page 84 of their online catalog) I believe it to be the same compressor that Extreme Air
http://www.extremeoutback.com/index.cgi?cart_id=6009521.4076&pid=30 sells for around $400 (compressor only!) that puts out 4 CFM (at zero head pressure). I bought mine for $169 a couple of years ago and have been very pleased with it. You could seperate the tank and compressor if you plan on hard mounting it, though. It puts out 3.4 CFM @ 40 PSI and 3 CFM @ 90 PSI and fills my 35s (and eveybody elses) really fast........Hans