Unenforceable. You can explore thousands of loopholes. Brazil has even more "strict" banking standard and it has a MONSTER real estate bubble. We used to hear the same crap: It cannot happen in Brazil, our baking system is ultra-regulated. 30% minimum down payment. Banks cannot operate as hedge funds. No loan made beyond market value of the collateral Yada, yada, yada.
Then you inflate the evaluation of the collateral. People took short term loans or family loans for the 20% down payment. etc...
Ya, maybe. Depends on the implementation? I'll admit, I couldn't imagine a law being passed in the US that would actually be effective and not riddled w/ loopholes as you mention (given the influence the financial sector).
Go Argentina-style? 50% down, 10 yr loans, 40% interest rates and an all cash market ?