SC2 has a special difficulty algorithm that allows for vast decreases in difficulty per 360 blocks (~200%) but only small increases (~14%). It seems that this could be exploited by using a script that does the following:
1.) Mines SC2 when ((number of blocks / 360) % 2 == 0), that is, every other difficulty difficulty adjustment period.
2.) Mines BTC when ((number of blocks / 360) % 2 != 0). This would allow you to use your cards for BTC in the meantime.
If a large number of people use such a script, for example 90% of the miners, the difficulty will quickly drop to what it would be if only 10% of the network were mining. This would maximize return per watt for people mining SC2, yielding bitcoin when they aren't mining SC2.
The difficulty won't drop, because miners with less than 1M SC can't mine trusted blocks. So it hasn't any effect on the actual blocks/time. You can decide to not mine SC, and the difficulty will drop a little bit.