I have seen a couple guys shooting Back Up Gun class using a fanny pack. As we all know, not every club follows the rules to the letter so I don't know if this was legal per the IDPA but it is done.
In shopping for a holster, it would be a good idea to simply look at the approved holster list on the IDPA web page. I personally do not use my carry holster for IDPA. My carry holster is a Milt Sparks Summer Special that I wear behind the right hip. While I believe this would be legal in IDPA, I don't think it is the safest way to carry a gun used in competiiton. The muzzle is pointing to the left when the gun is holstered and drawing it would result in sweeping the muzzle past anyone on the right and possibly directly behind me; like the guy running the timer. I am using a simple Fobus paddle holster for IDPA, worn in a conventional manner on the right hip. It is a legal holster and I think it is the safest method of carry for sporting purposes. I realize that many, if not most, of the people shooting IDPA do so in order to become better with their carry gun and I am not keeping with that line of thought. I shoot IDPA as a sport. I follow the rules and try to keep in the spirt of the whole thing, but I do use a different holster and gun for IDPA.