I have no reg to stand on (I learned my lesson yesterday I think it was) however I personally don't see the point in a cadet doing FRO training to begin with. At minimum if memory serves, you need FRO for AOBD at the minimum/soonest. AOBD is restricted to 21+, therefore Seniors Only, which I think is where this whole "FRO is Senior only" thing is coming from
EDIT FOR CLARIFICATION: Reviewed the SQTR. Actually there is no Age Requirement in the AOBD SQTR. Only need to be an MO or MP. So theoretically an 18+ "Senior Cadet" (18-21 cadets) as an aircrew member COULD do FRO. But it's up to your local command to approve or disapprove I'd say. As the FRO SQTR is literally one task "Just complete the training" it's up to your home command to decide if they'll let you or not
That would make sense, and I would have to problems with that. However, FRO training isn't in person: it's on LMS.
Basically, eServices made its own little rule that cadets can't do it. The same way Vanguard can make a rule about uniforms without anything in the regs to back it up (and look how we roast Vanguard for it).
Also, you can get PSC without AOBD, so while you are correct in saying that AOBD is the lowest qual with an FRO training requirement, it isn't technically the "soonest" one would ever need it.