Does the reinvirgoration from hearts of life stick?

[K]

Nope.

But, you can stack various hearts and only get the Chest Wound wound once. So, if you were putting a Black Heart into a guy, a Heart of Life can offset the +5 Encumberance with the +10 Reinvig.

Undead that aren't Lifeless can also take Hearts of Life and still not get any melee encumberance (though the +5 spellcasting encumberance still applies from the Chest Wound). It allows for SCs that buff x 5 then attack since the attacking is not creating fatigue but 10 is coming off each turn.

Also, the best Communion Slave is a Revenant (or other cheap undead mage) with a Slave Matrix. They have base 0 encumbrance, so each time a Master casts a spell the Slave won't take his own encumberance on top of his share of the Master's fatigue (which he can do several times a round if you have several Masters).