it doesn't address a "MIDI learn" style of binding, which i still
can't see how to implement since the action that would start it occurs
(presumably) with some user action in the plugin GUI, and the host
doesn't know about it.
The last SVN of A3 I tried actually does solve the issue in a sense. By exposing the parameter as an automation lane you can use Ardour's standard midi learn on the exposed control UI element that appears. Given this was essentially the suggestion I made in the earlier conversation, the implementation seems just fine to me...