Rinandien:
If GM asked you to provide a reference from another GM in whose game you are already in?
In the past, I've seen RTJs request a link to a game you're playing in with open threads and the character name you're playing. The implication wasn't that they were going to ask the GM, but rather take a look at your activity in that game and see how you've acted in the past.
In the end, though, it suffers from the same limitation of all things like this. Past performance isn't a reliable indication of future performance. PBP is a long term format, and it's rare that someone's life doesn't throw them a curveball somewhere along the way and their posting habits change.
SunRuanEr:
Sometimes, GMs and players just don't fit each other.
Presumably, if the player is giving the reference, they won't pick a bad one.
Of course, not every GM is going to want to answer some other GM's request for feedback on a player. Some people can be... prickly.