It seems like one currently has to mess with a script action to toggle enabled/disabled.
It should be straightforward to add a button that defaults to disabled (to only enable it once associated fields have some input).
I just finished up up adding a #button.disabled attribute. Further, dynamically enabling and disabling can be done with wf:enable or wf:disable or wiring an #enable or #disable action.
I'm doing the same with the readonly attribute. I'm going to close this issue. Thanks.