Volume and Deadheader
I was thinking along the same lines but with a rather different direction.
Pavlov ringing a bell and his dogs salivated as a conditioned reaction. For some pilots a lot of flying is conditioned reaction when there is a sudden workload increase with failures and alarms - it is the conditioned reaction that will win as the tunnel vision increases and the hearing fades to nothing.
In both AFR 447 and this flight the PF relatively inexperienced was suddenly dropped into alternate law. The roll becomes really squirelly and both PF fought to get wings level (tunnel vision) but their conditioned reaction from all their flying on the aircraft was that to go up pull back and the aircraft will go up. This is of course not the case in Alternate law. Pull hard back and the aircraft will stall.
So what cues are there to a PF with tunnel vision fighting for wings level that the aircraft is now in Alternate law and requires different handling. Almost none.
I would suggest not a stick shaker but perhaps some kind of 'stiffener' so the stick is less easy to move a haptic clue that you are now in Alternate Law respect the stick as there are less protections. This could also reduce PIO by slowing the rate of movement (less mayonnaise more thought).
Of course there will be those who had and have no problem with dropping into Alternate Law but there will always be some out at the end of the probability tail that don't notice and revert to type expecting protections that are no longer there. There will also be those that are perfect in the Sim but lose it in the real aircraft.
So a brake on the sidestick making it a little more difficult to move might be sufficient to remind them that they are in Alternate.