IIRC, the bottom of the aluminum stays anchor the belt to the ILBE.  I'd have to look at my ILBE to take measurements, but it seems to me like either of the two mentioned candidates could be possibilities.  At most, it would mean removing some of the stuff in the small of the back, and then sewing-on some webbing to mimic the arrangement on the ILBE waist belt.

Since you already have an ILBE, I would take careful measurements of the area where the aluminum stays pass through the built-in tunnels on the ILBE waist belt, and then call the two mfrs cited above and as their tech people if the other belts are compatible, or if they can be modified by the mft to suit your needs.

All the above said, the reason why most padded waist belts are free of PALS webbing is that the waist belts themselves are so bulky that attaching anything that would add depth to the belt, such as a pouch, would interfere with the wearer swinging their arms while walking.  Tape or tie some of the pouches you are thinking about using to your existing ILBE belt, and see what you think before proceeding.

Funny you should mention this, but I happened across some sleeves that would do so.  Surplus, and mde in CB by Eagle.  They would only fit on the webbing which extends from the front, padded portions of the ILBE waistbelt, and which are terminated by the waistbelt buckle.

Problem is that in  most cases, small MOLLE pouches with PALS webbing can attach directly to the 2" wide webbing belt, so there is really no need for sleeves.  Moreover, itr is generally considered a wise practice to keep the front of such belts unencumbered by anything except the smallest and thinnest of pouches.  I know of no sleeve that would fit over the padded portion of the waistbelt, although something could be made, I reckon.

MOLLE waist belt is a LOT thinner than the ILBE waist belt..  Again, try out things with temp attachments before making a decision.

Those OEM pockets into which the stays fit are put there so that the belt will carry a lot of the weight of the pack.  Absent a solid, load-bearing connection between pack and belt, the belt obviously will not carry the intended load, and perhaps much less.  Personally, I would not give up the load-bearing capability of the waist belt in favor of some PALS webbing (of questionable value, IMHO) on the external surface of the belt.  YMMV.

That's a frequently-encountered issue, because the pack's waist belt goes smack where most folks would wear their war belt.   Some folks make sure their suspenders for their war belt have enough length to the vertical straps so as to allow the war belt to be worn below the pack's waist belt.  Some stash the war belt inside or on the outside of the pack.