Can you cut-back the Salvias below the infestation and then burn the debris? I've never heard of this before.

I am not sure how you would treat it. I have a feeling it is only going to be common in areas that are in close proximity to wild sage populations. From what I have read and from the looks of the plants I observed, the plants are not killed but disfigured. Treating it would be a matter of personal preference I suppose.

I thought it was interesting to see that the plants are beneficial to many more creatures than just your standard birds and bees.

It seems to me that these gall midge infestations come and go without really affecting the plants for any length of time. I've only had a very small amount of midges on my apianas, don't remember seeing on the melliferas, with my leucophylla being much more susceptible at my location. Several years ago I had a one plant of a grouping of leucophyllas covered, it seemed, on ever leaf and branch. I thought I'd lose the plant. The next spring I checked it again and not a single sign of the midges, the plant appeared very healthy and none the worse for hosting the midges for a season.