As far as I know there is no foolproof way to do what you want. I could be wrong though, but since healing with keys is so unintuitive I've never dabbled with it.
I would recommend you set up Click Healing instead and use Click Bars for your Companions. You can even fake all extra mousekeys for your Click Bars by enabling/disabling a Key Map on Mouseover, thus allowing you to heal your Companions using the same Mouse Buttons as for your Party (even one extra since right-clicking will work).