You'd have to have some way to square the distance tools with the bar itself, and also with each other. But I see what you're getting at. The Leica measuring tool could be used alone, too.
Like Marc, I'm thinking of some of the following RRS or similar components.
Stuff that can be used for horizontal alignment/squaring:
- 1. MPR-73: 73mm Multi-purpose rail (with 1/4" camera screw), for mounting under the Leica Disto. It has an anti-twist flange that can be mounted to square the rear of the Disto. I had a Wimberley P30 lens plate laying around which functions just as well, maybe it's cheaper.
- 2. B2-FAB/mAS Mini-Clamp Package, for mounting the MPR-73 on a horizontal bar at a right angle.
- 3. A horizontal (CB-18 and/or CB-28) Bar, for sliding the miniclamp package with Disto along to check for identical distance at both ends (or average along the width of the bar if the wall is not even). For increased accuracy, the bar itself can also slide in the clamp that holds the bar, so you can achieve a measuring base of allmost twice the bar length.
Then you replace the bar by your camera with camera plate.
Following this horizontal parallel bar/wall calibration, you can calibrate the vertical parallel by minimizing the measured distance when you vary the pitch angle. It helps to use a geared head or a geared leveling plate for such precision alignment.
BTW, using a bar parallel to the surface automatically compensates for any minute non-square mounting of the Disto to the bar. Whatever slight angle there may be, the distance is measured at that same angle in the left and right measuring position.
To give an idea of the accuracy. A 1 mm difference over a 800 mm measuring base (like with a single CB-18) represents 0.072 degrees, and a 1 mm difference over a 1300 mm measuring base (like with a single CB-28) represents 0.044 degrees of accuracy.
'RRS' also sell a slightly cheaper package which combines the dual clamps with a bar (items 2 and 3 of the above list), in case you have don't have one or the other yet and would have to purchase both.