Why, continents represent a mere technicality, Thaddeus. Even doing a $20 challenge merely as a painting and display exercise would be fun and worthwhile. One way to do it would be to exchange armies with a promise to play and post photos and results on TWW here. My current rules of choice would be FFOT3 but Commander System is doable as well if slightly less granular.
The Modified 3×20 Challenge: one person paints and bases the army, the other plays it off against his own, either solo or with a willing third party as opponent, and agrees to post suitably purple battle reports and liberal photographs. If the constructed army wins, the recipient keeps it. If the constructed army loses, the recipient keeps it and reimburses the other’s Defense Budget for $20.
Frankly the $20 is just a gimmick and I’m willing to dispense with it entirely and just do an Army Exchange.
If more people than we were interested, that could be like Secret Santa: you’d know for whom you build and paint your army but not who’s building yours.
An idea anyway. You’re right that basing conventions would be proper: I’m perfectly happy to translate the FFOT convention of one vehicle=one platoon on a piece for piece basis: I frequently single-base my 3mm vehicles. I’m making 3/4″ x 1″ with a 1/4″ label my new basic convention for that.