The B777 has an alerting system that will progressively alert pilots if neither one touches a mike or knob over a period of time. The alert progresses at stages to culminate in flashing lights and loud aural warnings. That's a great idea!
On the A320, the under glareshield panel on either side of the FCU has six or so inches of unused panel at eye level directly above the primary flight display (electronic ADI), between the Master Warning and Caution lights and the Autoland light. This is precious, but unused real estate where I would incorporate the following two additional new lights:
A. A blue, selectable on/off light a pilot could use as a reminder light that he is in the middle of something like fuel balancing or an interrupted checklist.
B. A red light that could be addressed by company or ATC to turn on when attention is being sought, like Selcall. An accompanying loud aural signal would make it even better.
Such modifications, I think, would go a long way in preventing a lot of embarrassing and potentially dangerous incidents such as the one under discussion.