Tested using the /target partyN UnitId last night on my lil three man setup and it works quite nicely so now I have a series of macros which target the various party members based on their position in the party list - seems quite nice though the downside I found is that by using the following (for example):

/target party4
/cast Healing Wave ()

That if there's no fifth member of the group that it defaults to healing the last target, though I'd prefer personal that in that case it would clear it's current target first and then not cast unless the named target (in this case party4) was actually present/in range. I'm sure there's probably a smart way to go about this but it's beyond me. In terms of raid healing you could use the raidIndex N UnitID for targeting though obviously, as many people have already pointed out, if you're raiding and responsible for a healing role then unless it's a farm instance your primary attention should be on your healing character and ensuring that the raid is healed and whole with DPSing on your second char coming a very distant second.