You mean like...
1. Target the MT.
2. Set focus.
3. (You can do this through addons or just normal macros):
Code:
/cast [target=focustarget, nodead, harm][] some spell
Alternatively, leave the MT targeted and use stuff like this:
Code:
/cast [target=targettarget, nodead, harm][] spell
As a sidenote, if you like auto-targeting, I personally couldn't play my healers without long macros that let me heal by target's target or focus or whatever seems convenient like:
Code:
/cast [mod:shift] nature's swiftness; [mod:ctrl] some shapeshift; [mod:alt, target=player][help, nodead][target=targettarget, help, nodead][target=focus, help, nodead][target=mouseover, help, nodead] some heal