The biggest problem is the saddle to bar dimension. On my road bike, from center of saddle to the tops of the drop bar is 27.5" and to the hoods is 33.0". On the stock Boardwalk, from center of saddle to the flat bar is 24.0". So I need to get another 9.0" of reach. Ideally I'd also like to lower the bar by 2.0". In a perfect world I'd have drop bars, but to preserve folding, I might have to settle for bullhorns.
There are many different Dahon handleposts, and some aftermarket ones, so I think there are a few ways to accomplish this. I'm trying to figure out which is the best.
Another wrinkle is the weird proprietary Dahon rear derailleur and RD mounting. But there is an adapter to mount a standard RD hanger, which would let me put any road RD on the bike.
Doing some gear math. Looks like a road triple with 55/39/30 and a 11-24 cassette would make this an extremely versatile bike. 120 rpm in 55x11 is 36 mph; 60 rpm in 30x24 is 5 mph; the 30 would be a real bailout ring; the 55 would be for most flat riding; the 39 would handle most hills of less-than-sadistic grade. (If this gearing sounds crazy, remember the wheels are 20".)
I can get a used Shimano road triple at the bike co-op pretty cheap. Ditto a 8 or 9 speed Shimano 11-24 cassette and derailleurs.
Getting brake levers and shifters on bullhorn bars might be fiddly. I'm not sure there is an elegant solution. I've seen pictures of integrated shifters (brifters, STIs etc) on the upturned ends of bullhorn bars. Reverse brake levers are meant to go on the ends of bullhorn bars, but then you can't put a bar-end shifter there. You can stack reverse brake levers and downtube shifter using Paul's Thumbie mounts, like:
http://qph.is.quoracdn.net/main-qimg-aca2cc8c64c5992fefe5fdf8dcab7a48
http://img30.imageshack.us/img30/1337/handlebars.jpg
Oh, did I mention, I hate V-brakes? To remove the wheel, you have to remove the noodle. But if your brakes are adjusted to run veeeery close to the rims, you can't remove the noodle.