Definitely possible with redesign, but on the current frame, just dropping in 20" wheels, I very much doubt due to significant (50mm) lowering of bottom bracket, and reduced fork trail. On the other hand, however, if 20" and really BIG section tires to eat up most of that, should work. But that's a big jump, I can't see 90mm (almost 4") section height tires on 20"/406.

I think Helix went 24"/507 as a distinguishing feature, "better than 20", but yes, it makes more sense to do 26"/559, or 20"/406, but the latter may have frame sticking out beyond the tires, folded, and the former may not fit checked baggage limit of 62" girth.