I think that is a wonderful device, made in the days when watchmakers and craftsmen made 'computers' out of brass.

It will be compromised by nature of it using an obsolete systems for film speed and possibly aperture as well, but the principle of calculating exposure, rather than measuring it, is sound. The idea still has advocates:

http://www.fredparker.com/ultexp1.htm

The real challenge would be to make a 'modern' version with a familiar ISO speed range. No reason why it shouldn't be a useful tool. I carry around cardboard depth of field calculators and widgets to calculate bellows factor. Never feels right pulling out a modern spot meter with a wood and brass camera.

Not sure I like the 'app' on a phone idea, though. IMHO, If the thing can suffer battery failure - it ain't steampunk! Get the brass out and start filing ;-)