Divine Omnipresence + Vanguard of the Morning Star do allow you to give allies a bulwark aura sometimes. For whatever reason it is almost impossible to land on an ally. I had my cursor directly over an ally both of us standing still with no enemies (because for some reason you can target enemies (this has limited use to say the least)) in sight and it targeted me.

I think this combination on Bulwark of Dawn should prioritise targeting the closest ally to your cursor location or alternatively be at the location of your cursor exactly and follow whichever unit moves over the center first.