I've read about it but never done it myself. I'm guessing it would be a bit tricky but taking your time it could be done. Maybe you don't even need air, not sure how far down the valves drop with the piston at TDC.
Maybe you could fit some needle nosed vise grips through the intake and exhaust ports to hold the stem while inserting the locks. You just have to be careful not to damage the stems.