I think the issue with a Probables V Possibles, SoO, Best Backs/Reverse Forwards V Reserve Backs/Best Forwards is I don't think people will really care. It's a crowded rugby calendar as is.
Presuming people don't care and it ends up looking something like City V Country rugby league, then why would you hold it? Ask yourself, is it a better trial than the hours of game-simulation drills that professional players do?
Test coaches don't lack for data-points when it comes to selecting players or combinations, the usual hurdle is a lack of trust of the newbie and plenty of trust in the incumbent.