Like some others have stated, the prospect of positive healing for PM is alluring, but should there not be more drawbacks to it? Why not offer an enhancement in the tree that allows for positive energy vulnerability at the cost of some fortification and and negative vulnerability. OR, better yet, why not start the forms off with partial vulnerability to both positive and negative healing and 50% fortification plus immunity to poison effects (not dmg). THEN bump up the necrotic touch to T2 (or remove completely) and put an enhancement in it's place that gives you increased vulnerability to negative energy and decreased vulnerability to positive energy. 3 ranks for (65/80/100%) vulnerability to negative and racial increase to fortification and (35/20/0%) vulnerability to positive. Rank 1 gives immunity to disease, paralysis, and poison dmg; rank 2 gives immunity to most death effects (the ones that can't affect undead), fatigue, and energy draining effects; rank 3 gives immunity to stuns, dazes, and mind affecting spells (altogether what undead form gives now).
Alternatively this can be done in the cores, further emphasizing the idea that you are indoctrinating yourself into undeath while also providing more build options and a more give and take approach to the benefits that undead shrouds provide.
Keep in mind that while Warforged characters also start with 50% vulnerability to positive as well as imunity to energy drain, fatigue, and poison dmg, they lack the immunity to stuns, dazes, and mind affects. They also lack immunity to most necromancy spells. These are huge bonuses that should be accounted for when giving PM healing.