Does it make sense to anyone else that chomp tests and carps about receiving a read-only parameter, before it tests whether it would require modification?

Yes. It is well documented that chomp only works if you give an lvalue to work with.

I don't want to sound cruel but you want to modify core functionality for a very specific edge case. Good luck getting it implemented.

It's easier to go with what works. Use a temporary variable if you think that someone will pass you a constant.