There is a huge problem with using a FISO for "controlling" airborne traffic: he is not a CONTROLLER.
Exactly, hence why a new category/qualification would need to be created, though that shouldn't be an issue for competent FISOs. It wouldn't necessarily be controlling (OCAS!), but issuing slot times for approaches. Maybe we could even move on and do it the US way, trusting each other to remain in the hold until the aircraft ahead has reached a certain point on the procedure!
FISOs do of course have no authority over airborne traffic, but they can still request that traffic reports at certain points of a procedure. Just like an ATCO really
which is how most training approaches at FIS fields are run. Self reporting and situational awareness make all the difference, meeting a Jetstream taking up the same hold at roughly the same level effectively non-radio makes life a little more exciting than it has to be.