If somebody else posted this I would scoff at the idea, but from some of the posts you've made in the past you're one to actually follow through on something like this, so I'll take it seriously.

What is your budget for this project? A Yaris is lightweight, FWD, very high MPG possible, and a very new used one can't be more than 9-10k considering the state of the economy, the recent low gas prices and the fact they started at ~12k. Depending on how you do it, the parts on that list could add up to quite a bit of money. Done on the cheap, you're less likely to get the results you want. If the price of the project creeps up to even 1/2 of the price of the Yaris I don't think it would be worth it.

That said, car suggestions:
CRX HF- the damn thing already delivered crazy mileage from the factory. You might have to convert the fuel system- I'm trying to remember CRX swap info from 6 years ago so bear with me, but I think the HF had some weird fuel system and if you got too crazy with the build and EFI it might need to go. Really shouldn't be much of an issue given the overall scope of the build, and the HF was lighter than the Si which is always good.

Suzuki Swift 1.3l 4cyl- we did get these over here, right? Very light, very small but awesome twin cam (right?) four cylinder.

If you were willing to go 3 cylinder there are more options but they all compromise the "fun" aspect.

*edit* Do you put faith in those Singh grooves? I read a little bit about them a while ago and wasn't convinced. Didn't he prototype them on old VW air-cooled engines or something? The gist of it (IMO) was that the grooves probably worked on whatever old odd-shaped-combustion-chamber engine he first tried them on, but I didn't think the concept was as universal as he makes it out to be.