The encoder I found is a surface-mount unit, just a wee thing:
http://www.usdigital.com/products/aedr/
It would have to be mounted to a small board near the wheel hub, and be connected by wire to the display/power unit.
So far I've only found a two-inch disk, which seems a bit small for our needs, but will suffice if I can mount it to the bike wheel.
Weatherproofing will be an issue, as will encoder-disk clearance. This counter will be a semi-permanent installation, unlike the JO counter. It's more akin to Neville's calibrated-rim setup, with the advantage of the machine counting the partial revolutions.