I admit I haven't read much of Threetoe's work, so I'm not familiar with that, but I think having magic be costly enough to use would be balancing enough. If it takes a lot of some resource to use healing magic, whether to heal a cut or restore a severed limb or torn lower spinal nerve, then you're likely only going to use it for things that traditional healing can't fix.
That's what I'd prefer to see anyway. Healing magic can fix the same things that traditional healing can fix, and more (severed limbs or other permanent injuries, maybe even death), but the cost to use it is high enough that you'd only ever bother with things that your other doctors couldn't just fix. Having healing magic just speed up healing would trivialize other healing methods, unless it was something applied after the fact and only worked if the dwarf had been tended to already.
What the resource to use magic ends up being is the thing that is hard to determine. If magic ends up getting tied to deities, then it could come in the form of sacrifices, but if it's based on value it's too easy to make a few stacks of *crundle tallow roast* and fix up anything. That would require a lot of rebalancing on the trading side of things, as well as fleshing out deities.
With the way that necromancers work now, it seems likely that we'll see magic on an individual wizard scale, which is more difficult to balance. I think I could live with wizards requiring nice rooms and making demands and mandates, but that doesn't feel like enough of a limit. You need a constant resource requirement, I think, and mandates don't really cover that. Having individual cooldowns on their healing ability helps, but that may not be enough either, unless they can only heal like once per game month, but that ends up causing situations where the wizard heals Urist McScratched when you want him to heal Urist McBloodFountainTheGushing. If he can heal permanent injuries and deaths, then it's only delaying the inevitable before everyone is back to full power, so you might as well not make it take so long.
I don't know, I think maybe it's just too early to really guess how this should be done. There are a lot of systems this could be based on that would benefit a lot from further refinement or fleshing out, such as trade (value) and deities (as a source of the magic).