From experience setting something like this up 13 years ago, probably the most successful way for Antigua to do this would be to contract out to a commercial company (incorporated in Antigua, and with no US principals or assets; ideally as Antiguan as possible), for a set fee of $21mm/yr, and to keep warrants or other potential upside (say, profit sharing in years 5+ of a 10+ year contract?)

That way the company can make arbitrary amounts of money, with some value eventually accruing to Antigua, while remaining within the WTO definition.

The company could choose to value its earnings pretty much arbitrarily (either really high, based on retail price times number of downloads; or really low, based on bulk data transfer prices).

The big issue would be keeping international connectivity up -- while the WTO allows Antigua to do this, IIRC most of Antigua's Internet access is on a couple of cable systems owned by non-Antiguan companies (mainly C&W plc); the US could make life crappy for those companies. There are some international law issues with cutting off all communications and with the contracts for service on these cables otherwise, though, but it could be messy.

Ultimately it would be cheaper for the US to just give Antigua an extra $21mm/yr in foreign aid in exchange for them waiving their rights under this judgment. This could be buried in some kind of educational or bilateral trade agreement or something.