I also have used Formencode off and on for a couple things but haven't yet taken the big task to use a different form validation and scheme-building package for our bread-and-butter product, a dynamic survey-building tool.
We may eventually roll our own, but we're currently using FormKit to do it all and have had to build many work-arounds to fit our needs.
I, too, would like to talk to you further about the direction of formencode and how we could participate in its development and testing.
I will be at this year's Pycon so maybe a birds-of-a-feather meeting would be great, or even a sprint on the code would be good. What do you think?