Like Kevin and Eric said, it's easy enough to dowith a pancake-style holster; but I don't think you're going to accomplish it without punching holes or stitching. Even a snap is going to need a hole punched to set it. The easiest way would be to punch a hole above the existing belt slot, and use a Chiago screw-post to attached a folded over leather loop. I have made my In-cognito IWB with removable screw-on belt loops for IWB and belt slots for OWB wear.
But even though the basic pattern for my Enforcer OWB and my In-Cognito IWB are essentially the same, the wings on the In-Cognito are higher to accomodate attachment of the snap loops.
Here-in lies the problem with your concept: if using the existing belt slots as attachment points, or even mounting above the existing slots on a conventional OWB you're converting, the rig will probably sit too high to function as a good IWB. Most OWBs are slightly high ride. An Iwb needs to sit lower so it doesn't feel like it's going to tumble up out of your waistband, or worse yet actually tumble up and over your waistband.
Addressing reinforced mouthbands on IWBs. They are not essential when quality leather and other materials are used. I'll avoid tooting my own horn here and just go out on a limb and say I dare anyone claim that a Brommeland Max-Con, a Spark's Watch Six, or an Alessi PCH are not the highest quality, best functional IWBs; but none of them have reinforced mouthbands.
