If i was you I'd just try to very very slowly turn the motor over by hand and see if you can get lucky and have the piston shove the valve back up into the head. Using compressed air now isn't going to help as the valve is wide open. Just to note my piston stop didn't work either on my car when i was trying to degree my cam. I just threaded on a nut to it then put a tack weld on the end. I then ground the nut down so it was just small enough to fit into the spark plug hole. Then it worked great and i got my cam degree'ed in.

 

that's what I did as well.... I didn't want to crank it without anyone watching the valve, because for all i knew the piston would have been going down.... so I didn't want to turn it the wrong way.

here's a pic on how close i got..