I was thinking about starting up a relatively obscure game and one that I've had trouble recruiting for in the past. It's just not a deep well to fish from and it can take a long time to fill up a game.

That being said in poking around the site I noticed that there are a couple other GMs who have launched some successful and somewhat large games and I was thinking about reaching out to the GMs to ask if they would be willing to mention my game to their players.

I first and foremost want to make sure I'm not violating site policies but also just curious about questioning the community about cool or not cool. I could see both sides of the argument so I can't really figure out where I would land...

That being said, to the moderators looking on I definitely have not done this yet. No need to check my rMails or PMs. Nothing to see here :P

There's no rule against it.  Some GM's might be more or less upset at this practice than others.

Let's say that there's a GM with a game named Bob.  In general, I think it's the standard practice to only ask Bob to advertise your game when you've been playing in Bob's game for a while.  I could be wrong though. :)

If the GMs you're thinking about are running games in the same or a very closely related system, you might start by inviting those GMs to play.  Folks that GM obscure systems often wish very much for those systems to gather more of a fanbase and would often like a chance to play, themselves.  I'd save the "can you advertise for me" as a secondary question.

 For My part, if a Player  asks    I let them make  a post for thier game  in OOC thread.

 on my side of things, if I am in a game with good  posters and the game is dying,  I'll post in OOC if they want to come  to  one of my games, not only  might I get a good player, but if the  Old Game   starts  back up, we can come  back over  enmasse.

 I  won't post in a thriving game, for that.Because if its triving, its  a game I can play too!

I'd rather suggest you search for other GMs running your same (relatively obscure) game and ask them if you can advertise your game to her players.
Eventually you may end up having a kind of "community" where players (and sometimes characters) can move from game to game or be present in all games of that "niche" ruleset or setting.
I experienced it and I think it is very cool.

If a person playing in one of my games asks to advertise their game in the OOC thread, I generally let them. I've also recruited players from dying games in the same fashion that donsr has. If your intent is to message GMs whose games you are not playing in with the purpose of recruiting for your game, you should make sure it's the same system. I would never advertise a D&D game in a Shadowrun game, for example.

I'm usually fine with players advertising other games in my game.  After all, it's not like people will quit mine to join another one.   That's not how we gaming junkies work.  :)
I'll usually even make a separate thread for such ads.
The only exception I might make is if the game being advertised is something personally distasteful to me.