The Robes of Atom's Devoted are an interesting item, Once you get 900 rads, you'll stop taking any radiation damage, but it reverts your HP back to maximum (check in the pip-boy).
This mod, will instead keep you at low life. Once you reach 24.5% HP, you'll stop taking rads and will also stay at that radiation level.

I did this for two reasons:

1) To make it easier to use Unyielding armour pieces and;
2) To use the Bloodied weapon prefix.

I did this by modifying the armour item and a perk reference. I'm not sure if the immunetoradiation perk affects anything else.

Obviously requires Far Harbour DLC, as that's where the armour comes from anyway.