I actually got a response on the wow macros forums from Cogwheel (the macro pro) and he gave me this solution. Not sure which I want to use now though.

/stopmacro [target=focustarget, noharm] [target=focustarget, dead]
/assist focus

Depending on the complexity of macros this is probably overkill and waste too many characters so you could even stop at noharm.

EDIT: I realized this actually doesn't work because it then wouldn't cast either. As Cogwheel said we need a continuemacro command.