Healing seems to be the single biggest weakness of the UD system. Adding another mechanic for that defeats the purpose of the system and as you noted in your article allowing healing to be too powerful flips the dice back up the ranks too quickly.
Maybe the presence or absence of healing by class or something and add that in to the original math?
Like - starting equipment by class or character varies as to available healing or if healing can be used so if healing is present they start on D8 if not healing present they start on D6? Try and build it in? Or accept healing might be “overpowered” but make it more rare.
It’s a challenge, but to remove the “healing potion” would be a bold move and it might even be a good one. It would put into the world a healing time requirement, to let the dice heal 
Yes, as mentioned in the article healing could be a problem.
The best solution I can think of is: to move the UD size up/down, the roll must be under the UD target number. So healing is not guaranteed to restore (in effect a whole D) and so has the same probability as a wound does of changing the UD size … a bit of symmetry up/down.
in a way healing is then an “attack” on the wounded status. You could carry that resistance roll approach through to everything in fact. Would that work?
A really big or tough monster might be a D20, vs a player character’s D8, is that how this sort of thing works or am I missing something crucial?
Monsters are interesting.
I think if you want to transfer this to monsters you might need to write a new system (maybe)!!
PCs have some many ways of doing damage … I think it needs thought. I think UD can be used for PC’s more thought is needed for monsters as I think you might need to simulate how tough a D20 UD monster is.
Basically, I don’t have a good feel for ‘monster’ UD now. I think the trick is to break the connection with reference sources and just assign UD’s to monsters and see how tough they are (within reason).
As a rule of thumb, each additional UD adds a ‘hit’ they can take. Lower numbers clearly make them tougher.
So as a guide - add UD for bigger monsters; lower the target numbers for tough monsters.
Or, simply modle them against the Fighter - the medusa is about as tought as a 8th level fighter - so I’ll use that UD sequence etc.
Ramble!
:O)
I can see Usage Die as Health working really well for something like a super hero Ruleset since most super fights are either kick and deadly or long and drawn out with a fair bit of swing between the two extremes.
For healing, I feel like that could be signified by either only allowing a certain jump back to a certain maximum, or granting a flat bonus to each UD roll where even if fumble, the bonus keeps the total roll out of the drop range and keeps them in fighting shape. This healing affect can go away on a hit or after so many rounds of combat occuring.
This way, healing still cab be used, but is a temporary bandaid to save the pc for a round or so.
It begins to spread the madness, but maybe - fighting dice and health dice?
And for monsters with a “special” attack - fighting dice and a magic / breath weapon / death touch die?
I can see that, though that sounds a little bit like Savage Worlds since the stats and powers are driven on by the dice that are attached to the stats if the characters.
Though, it would be interesting to see a Dragon be scalable with something like UD determining the strength of its breathweapon.