Just did it. I'm in there right now. WOOT! Nice find mummy!
Here is a tutorial and video for how to get into the Temple Spawn Room.
Tutorial:
1. "Rocket Jump" on top of the roof of the doorway of "Hall of the Moon".
2. Walk down the hallway and look towards the green grassy area.
3. Crouch and roll off the roof and shoot a rocket immediately.
4. Try to move to the right and roll a 2nd time while falling.
5. Walk over to the 1/2 circle area and fall down and walk into the "Spawn Room".
* Steps 3 and 4 need to be done all together, otherwise you'll die!
Video: http://media.putfile.com/templespawnroom 
No banana's this time! I promise.
Enjoy the video! Thanks.