That's absolutely hilarious, and a pretty fun hack.
By the way, if I understood you correctly, this sentence
> I'm still missing 5 opcodes (MULS, MULSU, FMUL, FMULS, FMULSU), but until now I did not encounter a C file, the AVR-GCC emits these opcodes.
could be rewritten as
> There are 5 opcodes that are not implemented (MULS, MULSU, FMUL, FMULS, FMULSU), as I have not yet encountered a C file for which AVR-GCC emits these opcodes.