On an emergency tank, you're gonna go through that procedure?That's why a back mounted pony just won't work for me. I pull the reg with hose out completely, trace my hand down the hose to the tank it's coming from, read the contents and MOD on the tank, match the mod against my current depth (if not air) and then, and only then, do I breathe off of it.
So if you're somehow OOA and your buddy hands you a reg, you're first gonna swim around him to see the mod of what he's handing you, and then if you like it, you breathe?

If anything, I'll breathe, then check. But since a pony is not compatible with any multi-gas dive imo, there wont be such thing as "wrong gas in pony".