Been there and done it. Got the scars to prove it.
JK, Ypedal. But seriously, mixing wheel sizes up is a bad idea! I ran a 26" in the front and a 24" in the back. I ended up crashing while making a low speed turn. Wheel flop can be quite dangerous! One second your making a turn and the next the bike is falling over.
I've been thinking about this though: A bike with a higher front end would be much easier to make an effective fairing. Where a stanard MTB has you crouched over the handle bars would require a 2-3 foot tall fairing, a bike where the bars are at your chest would require only a very small fairing to cover the "pocket" of your body. I'd like to find a way to achieve this with out screwing with the handling of a bike. Because $ per $ and pound for pound, nothing can increase speed and efficiency like a fairing.
But I digress. You'll have no luck finding 20" or 24" wheels with disc hubs. Which means you have to lace them yourself. No biggie; it's actually kinda fun! However, it gets expensive buying spokes and hubs separately. Especially for odd sized spokes. I ended up scoring a free 24" rim and a cheapo disc hub from my local bike co-op, and then finding the correct sized spokes for only $20. However, if I was gonna do it right, I'd use a nice hub (sealed cartridges) and those are easily $50-$100.
I priced out building up 18" motorcycle rims with moped tires in this thread.
After (custom) spokes, tubes, hubs, and the rest It'll end up near $250. But according to some calcs it'll give a overall diameter of 23.6" with a fat ass 2.75" tire.
20" is too small for comfort. 24" sounds perfect right?