I certainly would not attempt this feat with x86 `asm` blocks :D. PTX is indeed very pedestrian: it's more like IR than machine code, really. All the usual "machine-level craziness" that would otherwise make this impossible is just unrepresentable in PTX (though you do run into cases of "oopsie, AMD don't have hardware for this so we have to do something insane").
It's a beautiful answer to a deeply annoying language feature. I absolutely love it. Yes, inline asm containing PTX definitely should be burned off at the compiler front end, regardless of whether it ultimately codegens as PTX or something else.
I'm spawned a thread on the llvm board asking if anyone else wants that as a feature https://discourse.llvm.org/t/fexpand-inline-ptx-as-a-feature... in the upstream. That doesn't feel great - you've done something clever in a proprietary compiler and I'm suggesting upstream reimplement it - so I hope that doesn't cause you any distress. AMD is relatively unlikely to greenlight me writing it so it's probably just more marketing unless other people are keen to parse asm in string literals.
