For what it is worth, I'm a metal fabricator, and I don't believe a pop rivet should secure the belly pan. Aluminum rivets don't have the strenghth to hold a panel upside down with the vibration, bumps, etc. Steel rivets will rust. If you have to drill out rivets, the hole becomes oversized.
If by chance part of the belly pan became loose during a run and drops down on to the salt, the results will not be good.
I think Keith Turk & Dave Freiburger had that happen to the HRM Camaro a few years back. The detach pan scooped up salt on the return road.
Seriously... using "Pop" rivets is a subject unto itself. The usual hardware store pop rivet has either a steel or aluminum body with a steel or aluminum stem. Their strength is low and the quality control is nil. They were never designed for structural applications but, if applied properly, can be used for non-critical applications. BTW, "Pop" rivets was (is?) a trademark of the Marston Corp.
Really good blind rivets are made by Cherry, Huck, Allfast, and a few others; Cherrylock or Lockbolt types are rated for structural use and they retain the broken stem under vibration so their shear strength is high. These types are available in a bewildering variety of materials and sizes. The highest strength materials are ones made of the super alloy A286 or a cupro-nickel alloy, Monel. Attention must be paid to providing the proper hole size and using the correct grip range when using any type of rivet.
You are right in that a couple of pop rivets won't secure a belly pan;... or much of anything else-- but aerospace blind rivets are in an altogether different league.
Regards, Neil Tucson, AZ