Sure you can! You just invert the pony and voila, you can reach the valve That said, you may have to get a longer hose. This configuration addresses the issue of the second stage free flowing and running out of air.A tank-mounted pony can't be turned on during the dive so it has to be on the entire time.
Rachel
To the OP, if your 2nd stage leaks air, there's a high probability that the 1st stage gets flooded. This has happened to me twice already. On both occasion, I charged my pony reg and didn't realize it was leaking air until the end of the dive, by then, the reg got flooded. Hope this helps.