After this thread I really don't know.
My instinct would be to say that if this is a rolling sub that means you can't scrum with 8 then you can't make the sub.
If it's an injury to #8 and you have forwards available to bring on, then do so, rather than simply bring on a back.
If you have no more forwards left, then bring on a back so you play with 15, and then we go to 7man scrums - both sides.
However I do see a whole can of worms in the way the law is written.
My position is based on analogy with senior law, which rules at U19 unless expicitly modified, and a general notion that the game should not be subject to chopping and changing like that.
As far as explaining the decision after the game ('cos during the game they shut up and do as they're told) then I have sufficient logic to stand my ground, they may not agree, and may push questions up the tree, that's their privilege.