While the IDPA rules used to refer to an "approved equipment list", I guess administering this proved to be burdensome and the 2005 IDPA rules
describe what is permitted and what isn't rather than listing holsters by make and model number. A July addendum doesn't seem to change the holster language significantly.
The IDPA rulebook is available here -
http://www.idpa.com/Documents/IDPARuleBook2005.pdf
Here's some sections that might prove useful -
F. Can have no offset backpieces and/or belt slots. The holster
may not offset away from the belt and/or body. No gap is
permitted in the following areas:
1. From the body to the inside of the belt.
2. From the outside of the belt to the inside of the
backpiece and/or backside of the holster.
3. From the outside of the backpiece to the inside of
the holster.
If you look through the belt slot area of the holster with it on
the belt/body, you should not be able to see any daylight. If
you can see through this belt tunnel area, the holster is not
approved.
Holster/backpiece must be constructed of “normal thickness”
common holster making materials, no filler is allowed to hide
an offset. In simple terms, the back of the holster must be held
tightly against the outside of the belt for proper concealed carry
(for questions, refer to #1 above and re-read the “Purpose”
section in the front of this rulebook).¹
...H. May not position the firearm where the breech face (autos)
or rear of the cylinder (revolvers) is below the center of the
belt. NO drop loops are permitted.³ Holsters for females may
position the breech face of a pistol or rear of the cylinder of a
revolver up to 1 ½” below the center of the belt.
NOTE: IWB style holsters are exempt from this criterion.
I. Must hold the firearm positioned on the body so an object of
¾”width cannot pass between the shooter’s body and the inside
of the firearm when the shooter is standing straight and
upright.² ...
¹ NOTE: Holsters for females are exempt from F. criteria, but may
not cant the handgun away from the body past 90 ° to the ground.
² NOTE: Female shooters are exempt from this test.
So, female shooters can use a holster with a drop loop (provided the breech face isn't more than 1-1/2" below the belt) and the 3/4" test doesn't apply.
My wife is still looking for a holster with a drop loop, but she's doing OK with a pancake holster worn with pants that fit lower on the hips. This sort of solves the problem of the holster canting the butt of the pistol into her ribs, but she'd prefer to wear pants with a higher waist.
I hope this helps.