So after rendering and implementing it, this is how it turned out.
I honestly think it works pretty alright! It's not too noisy, they're similar but still visibly distinct, and the clear colour-coded role symbol leaves no doubt to be had. I had one thought to make it permanently show across unit frames as per active spec, but my biggest hurdle there is that I'd have to figure out a good way to determine spec for hostile players. And inspecting hostile players is a no-go, right?
Still, as far as tank/healer/damage role icons go, I think it's a pretty swanky solution.