i modified my macro for a raid once.
my normal healing was something like
/castsequence [target=focus] xxx,xxx,xxx

however for the raid I changed it to
/target RaidTanksName
/castsequence xxx,xxx,xxx

then I changed it back when the raid was over.
I would think you could write a new macro , one for party healing and another for raid healing
and edit the raid healing macro to handle the new raid tank names.
/target Foo
/target [noexists] [dead] Bar
/cast xxx,xxx,xxx

you can also limit your healbot to only show the tanks or only show the groups in which the tanks are in. I limit healbot so that it doesn't show 25 names, it shows 10 or 15 names which includes the tanks group and my group if I'm not in his group.

and last you can always target the boss and heal anyone he's targeting
/cast [target=target-target,noharm,exists] [target=target,noharm,exists] xxx