I dont think it would be too hard, if you were nifty with 3d programs and pep and balsa wood you could make urself a nice warthog/scorpion; you have converted IG for marines; with IG you even have rules in place for ODST orbital deployment, etc; largest difficulty is making 40k version of AR (model, not rules)
There was one on geocities a while back, not uber-well-converted, but this might be the same one i couldnt find what you were talking about on ebay...