well if you have your focus set your your main... you can have this

[code:1]/cast [target=focustarget] Healing Wave[/code:1]

or even

[code:1]/cast [target=focustarget,help] Healing Wave, [target=focus] Healing Wave[/code:1]
that last one is untested but should work by healing your mains target if it is friendly or healing your main if it isn't