Mysteriously (ahem) my hayfever developed AFTER I went aircrew. You may not be able to actually DO that, but it worked for me. Once they've spent a load of money on you, they view it differently.
Mind you, I trained as aircrew in 1973 when standards were a lot lower (I ACTUALLY meant medical standards, but perhaps it was a Freudian slip).
Even now I take non-drowsy medication which was prescribed with the full knowledge of the CAA, medication is Cetirizine (no I'm not suggesting you take it, just that 'acceptable' medication is available)
Good luck