This sounds like a trivial hijack hack, if I understand it right.
You want mute_on to "save current source, then switch to AUX", and mute_off to "restore original source". Or possibly with mute_on/mute_off reversed.
I can implement this in about 5-minutes as a pair of virtual button codes inside Hijack, if it would help.
We just need to agree on good names, something like "save_source_and_switch_to_aux" is a bit wordy for my liking.
Actually, I could just implement "save_src" and "restore_src" button codes, and then one could define a macro to do "save_src + aux" for the "mute_on".
???