Repair Deep Plaster Damage on Outside Corner
I think I'd be installing modern corner bead. I expected to see an early version of it in your corner, but there's apparently nothing. 

Grab a stick of corner bead and some long drywall nails. Pre-drill the plaster if you have to, then nail it on, making sure to get good penetration in the framing. 

I'd use a setting-type joint compound for the rough fill to add toughness, then skim with your favorite joint compound.