MrHorgy
I'm all for flight safety, but I think this is a bit OTT. It will cause frustration on the R/T for one.
I really do not understand from your post what this SOP would achieve. Your post reads as if the PF can change level or heading when the PM is not listening in, as long as the PF has reconfirmed with ATC the instruction. Is that correct? That's how your post reads.
A few points spring to mind immediately.
First, the PF has to readback the instruction to ATC, therefore any mistake would be caught (in the vast majority of cases) at this point.
Secondly, I would have thought that the point where a mistake is made is not in the actual understanding/hearing correctly what has been instructed i.e. 'Climb FL130', but when you actually dial that into the aircraft.
Carrying out the action incorrectly will not necessarily be avoided by reconfiriming the instruction with ATC before you do it!!
Thirdly, even though a good safety culture is to be applauded, I would be a bit peeved as a highly trained pilot that my company did not trust me to do something as simple as change heading or level if If the PM was not monitoring my actions!!