I've proposed a similar rule, where when affected by healing that takes an action (Cure Wounds, Lay on Hands, drinking a potion, but not Healing Word) you may spend HD equal to the dice of the effect. Lay on Hands and other flat amounts counted as one die for every 5 HPs.
I did dice instead of spell level because (a) not everything has a spell level, and (b) I wanted a Mass spells not to have as huge an effect on each person as an equal level single target heal.
This was in conjunction with allowing an action in combat to spend HD up to your proficiency bonus once per short rest.
What some seem to miss is that moving HD from out-of-combat to in-combat makes no change at all for total potential healing during the day, just makes it more accessible in combat. And right now in-combat healing is considered worthless except for picking up fallen PCs.
Plus HD recovered is one of the only two standard beyond-long-rest attrition (the other being exhaustion levels), which too much of either cna carry over to the next day. Which I like.
In retrospect, I should probably boost Life cleric, as it's ability would become a much smaller percentage of the total.
BTW, I don't give out a lot of consumable healing, nor allow unlimited purchase, so that's already in place for me.