After enchanting the manasteel chest and pants with thorns 3, mob attacks would count towards the durability on the armor directly. This happens with a full mana tablet in my inventory.

MC 1.7.2
Forge 1082
Botania Beta 31

I looked around and asked around, doesn't seem like there's a workaround to this, sorry.

Interest, I think this is a vanilla bug. I recall it used to affect IC2 jetpacks. I'll have a look.