I've been thinking about getting a glue on block heater for my Ninja. I pick up a 200watt magnet mount heater to use over the winter last year(Sits outside all year). Found out there wasn't a bit of steel I could stick it to. So I just kinda wedged it in there. I don't think it really did much good. But this year seeing as I do a lot of riding at night (been getting down to about 40's lately)and started to read Ecomodder, I went ahead and did a partial radiator block (painted cardboard). I've noticed a considerable increase in performance and my mileage has gone way up. I do a lot of short trips 3-6miles and I've found it doesn't like running low RPM's (about 3000) when it's cold.
So i'd say go for it
One thing I was wondering is if you could design one that could be battery operated 12v. It would gives the bike an extra boost of heat just before/until it's warm? That way you could use it when your out and about as well. I don't know how big a pack you would need to do something like that. But I think the increased warm up would help when it comes to mileage.