Apart from giving him a bit of a head start in terms of learning the playbook whats the difference between doing this now, a futures contract towards the end of the regular season or grabbing a developmental guy on day 3 of the draft.
None of them will get any reps outside the PS. If you want to add an developmental guy on the PS be my guest, but most positional coaches are working hard on QB1/QB2 and gameplans to the point that there won't be much 1 on 1 time left for additions.
I am not entirely sure what you are realistically expecting to come out of this. It is not like McDaniels and BB have time to sit and was tape with the guy or whatever. That's the advantage of the offseason where the staff doesn't invest every available minute into gameprep.