Oh, the S-hooks should be tuckable, yes, if you mount the very bottom of the tab to the holster. If you mount anything up at the top of the holster, it wouldn't really be "tuckable" because you can't slide your shirt between the loop and the holster.
I don't think the belt slots will provide the stability you'd want for mounting your loops or straps. You may tighten down on the screws as tight as you like, but there's still almost certain that the screw will slide up and down in the slot, and/or that the slot will gap open just a bit and the screw will just pull through.
I guess if you use a big fender washer behind the screw head that's not as likely to happen, but I'd much MUCH prefer drilling a small hole so I don't have to worry about it.