Excellent exercise, excellent plan. We do something similar, one diver at the bottom of the pool, the other swims two laps hard, surface dives (no regulator use) and approaches the donor from behind. The OOA diver taps on the donor's shoulder, signals for air, the donor counts to three and surrenders the primary. The staff felt that this was a pretty close simulation of what "the real thing" is like, and the students' awareness of where their buddy is and what their buddy's doing shot way up.
We do something similar at school, although not quite to that level. We break the class up into 2 groups and go to opposite sides of the 25-yard pool. Both groups drop down and hover (in ~3-4ft of water) and wait for the signal given by the instructor or TA.
One of the two students waiting on either side of the pool is thrown a signal that they are OOG (student is randomly chosen so they don't know it's coming). They must then give the OOG signal to their 'buddy' (on the opposite of the pool) and they start swimming towards each other. The OOG diver has to take their reg out and clip it off as they're swimming. If they forget to give the OOG signal, their buddy doesn't move towards them or do anything else until the signal is thrown...more than one time, that's made the interesting situation of having the OOG diver swim almost the entire length of the pool underwater to get air, because they forgot to signal they were OOG.
Thanks for these exercises, folks, I'm going to try them out.