Well, it seems logicial, if you're still locked on the tank, casting on <t> will cast the spell on him.

Dunno if a <mo> (mouse over) would work if you highlight someone in the list, but it worth the try.

If it doesn't, my advice would be to change your system, instead of staying locked on the tank, focus him, then create healing macro for your focus <f>.

Then you'll be able to switch to whoever need you in your party, and still directly cast on the tank without having to lock him before.