There is a easy solution for Tesla. They could integrate a trickle charger into the charge path. When a tesla is plugged into AC it would float charge the 12v and offset the vampire.
A high quality float charger is $150 max.
They recommend keeping the car plugged in when possible anyway.
Of course addressing the vampire is the preferred solution.
The way it works now is that the car is the master of the charging circuit. The EVSE (HPWC and UMC are Tesla's versions of the J1772 standard with a proprietary connector) simply advertises how much current it can supply via the pilot circuit, and the car commands the EVSE to close it's contacts and supply power. When the car is done charging, it commands the EVSE to open it's contacts to interrupt the supply of power. There is a safety issue here; the plug at the end of the EVSE charging cable is not energized unless it's plugged into the car and the car commands the EVSE contacts to close. That's how scheduled charging works too; the car controls the EVSE to supply power at the programmed time. Just adding a trickle / float charger on the car charge port with the current charging architecture would be worthless. The trickle / float charger would not be powered unless the car is charging, but when the car is charging the DC-DC converter in the car is charging the 12V battery anyway so the trickle / float charger would be redundant.
Now, Tesla could design a change to the charging architecture. One way would be to include another set of high current contacts in the car under software control to isolate the on-board charger from the charge port. This would not require any change to the EVSE. The car could energize the EVSE charge cable the moment it's plugged in and de-energize it when it's removed (safety feature), and control charging the HV LiIon battery via the additional high-current contacts in the car. This way, the change port would be energized whenever the car is plugged in to the EVSE, but the car could control start and stop charging internally. Under those conditions, a trickle / float charger from the charge port to the 12V battery would be effective.
Since the car can control the magnitude of the charging current, it might be possible to add the trickle / float charger and change the software without adding any additional hardware to the car. In that case, energize the EVSE when the cable is plugged in to the charge port, but reduce the current to zero until the scheduled charge time is reached and when the charge is complete, but leave the EVSE contacts closed to energize the trickle / float charger while ever the EVSE is plugged in.
Of course, both of these schemes waste power in the EVSE holding the contacts closed when the car is not charging.
But, I agree wholeheartedly that significantly reducing the vampire drain is the preferred solution. 40-50W running continuously is simply enormous, and that's why the 12V battery is under such stress.