I don't think it would _keep_ prices too high. Make it a one time deal. 1% re-fi's for all homeowners during 2009 and 2010, for example. It stimulates the economy, and interest rates return to normal market based rates after the period expires. The rates may actually be a little lower, since demand will be off. That's good for business, and not inflationary. Allowing new home purchases during the 2 year window would be fine with me too, but those buyers will see a steep loss of equity as the period draws to a close (while re-fi'ers would likely see a temporary "bubblet" in their home values that would end as the window closes, but having a short term "bubblet" right now certainly wouldn't hurt anyone).
100 year mortgages may be a good option to have, but I would never get one or reccommend one to anyone. For one their interest rate would be quite high. Also, You probably wouldn't build any real equity in your house for the first 60+ years or so, and the holder would be way more exposed to downside movement in the home value. The mortgage interest tax deduction would be nice to have for 85 years or whatever, but that doesn't get even close to outweiging the downside.
The only reason I could imagine a 100yr fixed rate mortgage being good for the holder is in a sharply inflationary environment. Maybe if they could be structured as the 100 yr fixed rate, convertible to a 30 or 15 yr fixed rate mortgage (at market rates, so sort of a put on inflation) at the borrowers discretion. This would be good for financially sophisticated and disciplined people, but likely bad for society as a whole. you can't really discriminate that blatantly against <del>stupid people</del> people of modest intellects, and many would get sucked into the poor financial choice of having too much house & mortgage, never convert to the shorter term, and get stuck with a poor investment.