Have you checked out the McSorley designs, he offers various larger versions of the original Loscost design, but they are for live axle (sure you could sort that out, though).
http://mcsorley.net/locost/
I'd be surprised if there was much call for larger chassis, though. Some call, but not very much. Then there would be the problem of the bodywork, especially the nosecone. Luego were offering the Viento, which is like a scaled up seven, fit any enigne or person, but think I heard somewhere, could be on LCB that they might not now be available.
Regarding the height of seat thing, it would need to be done in such a way that the upper seat belt mounts were IVA height above the seat (taking the seat out for the test is not much good due to still needing headreasts)