The good news is that HD Tracks got back to me very quickly. The bad news is that they don't know. They received a 24/96 set of files from the label, and that's as much as they can confirm.
Given how important mastering is, even on a direct digital transcription of the original master stereo tapes (assuming for a moment that that was what was used), that is a drag.
So I think I have to engage in an experiment, and pick up the 45rpm lps coming out in December, and compare with one of the cheaper single tracks on HD Tracks, and see.... and then make the assumption that other Impulse 24/96 material on HD Tracks will have been sourced and mastered similarly.
Why use the 45rpm lp as the benchmark? Because the entire signal chain and mastering process is well documented on those, and from experience seems to pull more out of the master recording than any other re-issue. So if the HD Tracks file has similar quality, I then become very confident in picking up many of the Impulse recordings they are starting to carry.