Loctite 3609 (trade name "chipbonder") is specifically designed to hold SMT components in place for reflow and wave soldering.
Most epoxies should be okay.. that's what the FR4 laminate is made from after all. If you're not sure, you can run a few drops through a dummy heating and see what happens. Small strips of Kapton tape would also work.
You could also look at thermoset plastics. I don't know how FIMO or Sculpey would stand up to a reflow cycle (they harden at 110-130C), but again, you could do a dummy batch to find out.
When you void a product warranty, you give up your right to sue the manufacturer if something goes wrong and accept full responsibility for whatever happens next. And then you truly own the product.