I've seen (don't remember details, sorry) some stuff from a European company, I think they may have been Swiss, not sure, but they seemed to have several solutions for driving from a manual chair, including some lockdown systems that didn't look like they used the same low-hanging bolt that the power chairs use - I think it was a sort of hook on a scissors jack that came up and grabbed the axle tube, or one of the other cross-members.
They also had a sort of head and backrest deal that hinged on the door pillar behind the drivers position, sort of like a gate. After the driver wheeled his chair into position, the thing swung into position behind him and provided a more supportive back and a headrest that worked in addition to the existing chair's back... Looked like a clever idea at any rate....
Correction to the above, checked my bookmarks, and found that I'd saved it -
http://www.lokkup.com/Default.aspx Not quite as I described it, but close enough....
ex-Gooserider