Chris,
Did you ever considered David Ek's setting circles?
They can be built quite cheaply.
With encoders like
this one (using upgearing, easy to do with timing belts) and with cheap old laptop (running CdC with ASCOM driver) you can have good enough push-to system....
Another source of (free) incremental optical encoders are old printers.. but you gave to be prepared for lots of DIY fiddling...