John,
Since this post is DIY area... have you seen
David Ek's encoder interface web pages?
Or
Mel Bartel's webpage? It uses mouse driver for interfacing encoders.
With mouse interface, or with Ek's setting circles this system can do much more than Argo Navis (GoTo included)... not to mention it is way cheaper.
Also, have a look at
OnStep or
SoundSteper webpages.
AstroEQ is also very interesting design.
I have used all of the above, except AstrEQ... because OnStep is good enough for me - it can run on it's own (no PC needed.. so not much stuff to carry around). However it has ST4 interface, and it understands LX200 protocol. No encoders yet.