I've thought about this, too. Great topic.
I think there are a ton of people out there who have floating-rate mortgages and who would love to lock in a rate, but they can't because their credit/income is bad or the house has fallen in value.
What people really need is an interest-rate swap, which Bryan is trying to create individually using futures contracts.
As has been pointed out, for so long as the rate outlook is "higher rates ahead," doing this could result in not being ahead financially when the dust settles. However, that's actually OK in times when the yield curve is normal, since if you're trying to lock in any rate -- be it through the mortgage market, the futures market, or wherever -- you should fully expect to pay a higher fixed rate than today's short-term rate.
I've always been a little surprised that one of the big investment houses hasn't offered this product to consumers. It would be SO easy for them to offer interest rate swaps and pick up spreads along the way. I think the problems are that (1) most consumers would not understand the product, (2) the public perception of the big, evil investment bank trying to seduce the public into any swap contract could be bad, and (3) psychologically it would be tough for a consumer to start writing checks on Day 1 of the transaction to the big, evil investment bank for the delta between the variable rate and the presumably higher fixed rate, never really knowing (or understanding?) that the big, evil investment bank would have to write checks back to the homeowner should variable rates shoot up.
And of course, the easiest solution -- for most people -- to get a fixed-rate loan is just to refinance into a fixed-rate product, and the banks love nothing more than refinances because they generate lots of juicy fee income, ergo they have little incentive to solve the problem by doing interest rate swaps.
I love this question, though. It's something I've thought about trying to implement. GMTA, Bryan!