Yeah, i presume the rule was brought in to stop a team bringing in a short term star player on megabucks and it not counting on the cap so as you say if they have been registered as trailists they'll have to upgrade them to full time players to play first team. For info the rule states....
'C:1:1:21 Trailists are prohibited from participating in a Salary Cap Relevent Match or any other Challenge Cup Match'.