I think the issue here is not as much about getting a larger driver to fit, but more about getting a properly designed enclosure for the drivers being installed. For mid/sub - bass, the enclosure design is just as important (if not more so) as the driver itself. For example - take a look at the JL 6W3v3 6" sub. The driver could be mounted in the lowers with some minor glass work - but just because it can be mounted doesn't mean it will produce the expected performance.
Eastside Harley designed (properly - I assume) an enclosure for the 6W3v3 sub to go in the right bag. It takes up ~1/3 of the interior bag space and must have a vent port drilled in the bottom of the bag. Point being that there is noway to get that amount of enclosure area in the lowers.
Bottom of page here:
http://www.harleystereo.com/custompage.asp?pg=customstereoNot saying it can't be done (I'm looking into it myself) - just saying there is a lot more involved than getting the driver to mount physically.
Keep us posted with whatever you come up with!