Oh my - so more than one of you have bought the $250 H.D.I. tool, for your own personal use? Wow! Especially if it only works optimally on some blade types for a very short while, as you have indicated.
As far as using the square is concerned with vertically side honed (e.g., dovetail cut) blades, I assume that there would be a slight angle between the side of the blades and the side of the square that fits against them. E.g., for dovetail cut, there would be a small airgap as you get further from the edge, but not next to the edge itself. If you apply the square to both sides, the size of the air gap should be the same.
If your eyesight isn't good enough use a good magnifier, with little distortion - perhaps a good quality Hastings triplet - though you would have to brace the magnifier against the square and blade so the distance would be the same. If you really want to go whole hog, you could get those cool looking flip-down surgical or dental magnifiers (I've not tried them) - though the best of those cost even more than the H.D.I. tool.
How good a job does the H.D.I. tool do of measuring rocker, and how does that work?
A shame that, based on what you have said, it might not work with Phoenix blades.
I'm sorry that I just assumed that Pro-Filer could do the job. I should take a measurement on the old Berghman sharpeners, to see if 5 mm is too small. But I don't love the stones that came with the Berghmans, and they only supported 1/2" ROH. Maybe Edge Specialties could make a Pro-Filer designed just for them? At one point, they offered to make custom gap sizes for different width blades.
BTW some of the best skate techs are very proud that they figured out how to sharpen Gold Seal blades, and other vertically side honed blades, with even edges, and also managed to avoid uneven blade width as you move along the length. (It would be very easy to accidentally give them a wavy width pattern, by not taking the same depth of metal off all parts of the blade.) But those skate techs are few and far between. I have wondered whether the putative benefit of vertical side honing is real, or whether it is just an excuse to charge more for the blades. That said, some high end kitchen knives have vertical side honing too - thats what they call "hollow ground", but the hollow is in the sides, not the bottom.
Anyway, I guess we have come more or less to a consensus that the claimed ease of edge maintenance on Phoenix blades may not be altogether real, unless perhaps you have the skills and the proper equipment. I personally wouldn't want them, for that reason. I LIKE being able to sharpen my own blades - and have chosen them accordingly.