I was also using SimpleAssist before 2.0. I don't think the same thing is possible anymore, as addons can no longer target things. As a less-than-optimal solution, I made one macro with "/set focus" and another with "/assist [unit=focus]" and bound them to the same keys I used for SA. Of course then I can't use focus for an actual focus, but it works.
A way this could (possibly, I think) be done would be to make an addon that (out of battle) would edit a macro and set whoever your target was as the assist and then just add your macro to the bar or use a keybinding addon to directly add a keybinding to the macro itself and during battle it would queue your changes. I'm quite sure this could be done according to my fiddling with 2.0 but *coughs* don't quote me on this cause I'm not 100% certain.