I am about to go to bed so I don't have time to post the exact macro I used but it should be easy to make.
I noticed this tonight on heroic halfus. If you put one target on focus and have another unit targeted you can charge and skullbash your target but also interrupt your focus. You just have to set a macro that does skullbash on your focus first then on the next line make it cast skullbash on your target.
I have not been able to test this to see if it is actually interrupting your current target, or if it only interrupts the focused target (I was using it to dps \ hold aggro on a drake while interrupting shadow nova), but it definately applies the skullbash debuff to both targets. Not extremely useful in PVE, but I could see it being very very useful in PVP, even if it only causes the 10 seconds of increased mana cost.
I will post back after I do some testing.
Also, this could very well work with any other ability that is off the gcd. I am going to do some testing with maul, heroic strike, etc to see if it is just this one ability.