Topic says it all. I know 20" is pretty limited for gearing, but can it handle SRAM's Dual Drive?

Bike Friday uses them on folders with 20" wheels.

Topic says it all. I know 20" is pretty limited for gearing, but can it handle SRAM's Dual Drive?Yes; standard fit on HPV's GrassHopper (406 wheels), for example.
Is 20" really that limited for gearing - my Mistral gives me 22" to 103" with just 2 chainwheels (39, 60) and an 11-34 cassette. A dual drive would give you a similar range. Guess you might want to go lower on a trike though?

Yes you can use it o a 20" drive wheel. In fact, they were pretty common a few years back. Used in conjunction with a standard 3x8 or 3x9 derailleur arrangement, you end up with all of your current gears, plus all of your gears in underdrive, plus all of your gears in overdrive. 72 or 81 gears. There's a lot of duplicate gears, so it's not something you use a lot during the course of normal riding; but it's nice to select underdrive when approaching a large hill climb, or overdrive for that monster downhill. I had it on my V-Rex24, and it gave me a 17-144" range.

Squeaker, your bike actually shifts with a 21T difference between rings? I know once you get above 53T all bets are off anyway, but Sh****o are designed for 12T or less between rings.

Topic says it all. I know 20" is pretty limited for gearing, but can it handle SRAM's Dual Drive?

Yep.

Another item you may consider is: a mountain or speed drive from Schlumpf (http://www.schlumpf.ch/)

Bike E used them also.

Squeaker, your bike actually shifts with a 21T difference between rings? I know once you get above 53T all bets are off anyway, but Sh****o are designed for 12T or less between rings.Yes, I was surprised too, but it works OK and, practically, I only need the 39T for the bigger hills so don't need to shift that much anyway ;)
As you can see from the pics, it's a Deore LX shifter, and I don't think you could go much wider either, as the clearances are getting small.....

It's used with a 16" wheel on the Bike Friday's original SatRday, and on all their other 20" bikes.

Topic says it all. I know 20" is pretty limited for gearing, but can it handle SRAM's Dual Drive?

I have 2 of them.

A 20 in overdrive is the same ratio as a 700 in direct drive.

My BikeE CT has a 3x8 SRAM Dualdrive with a 20" back wheel. I believe that the chainwheel is 44T. Don't know what the cassette has on it. Works great.