Whether you:
a. force 0.8 to reject valid blocks which it predicts 0.7 cannot process
b. force 0.8 to not generate blocks which it predicts 0.7 cannot process
What about b. with an expiry date of say 60 days?
So 0.8.1 is released compatible with the predicted behavior of 0.7, and miners are asked please upgrade to it within the time period offered.