Ok, it'll work with the removable push type clips. The thickness stack up doesn't seem an issue at all and I've drilled out the rivets on the leading edge and everything seems stable/secure. These are the push type clips I'm using:
However, a second issue is this: I can't get the car up enough to drill the new holes (about 8 mm dia, .31") in the trailing edge of my panel. I have an angle attachment for my dremel, but it will only chuck 1/8" max. And I need to chuck .31" to drill the new holes for the push type clips. I have maybe 9" to play with; at the moment I have no means to jack the car up (the dremel angle attachment was the workaround for that in the past). Any ideas? I can look in to detaching the panel my underfloor infill panel ultimately attaches to and thus can pull everything off and drill off-car. Just not sure I can reach all the detachment points for the main panel. Guess I'll have to have another look at the situation.