Ended up using this piece here -
https://competitionmotorsport.com/collections/sub-strap-mounts/products/cms-performance-sub-strap-lap-belt-mount-gr-supra-gr86-brz?variant=47863616930011
FYI, comes with no instructions. Would highly recommend cutting the carpet and not pinch the material.
Did this ever come to a supportive conclusion? Similar boat here for other reasons. Picked up BK bits for E46/E9X thinking it would work to clip the lap belts in. Held pieces up to Supra seats and it looks like the garnishes and OE belt pieces create interference / problems. Going to give it a...