Part of the answer:
Dimmer Extender PluginThat plugin allows you to catch Start/Stop Brightening/Dimming Insteon messages from dimmable Insteon devices. So, if there were corresponding start/end brighten/dim actions for the target device(s) then you could pair them via triggers. It's one reason we added those actions for Insteon and Z-Wave devices in 7.3.
We haven't directly added those Insteon commands to Indigo because, in all honesty, we were concerned that a corresponding stop command might be missed due to signal issues with Insteon. While just brightening/dimming a bulb might not be a big deal, some people might use it for other purposes where it could be damaging if the stop message might be missed (think volume control where the volume starts increasing and just continues to go up until it blows out a speaker because the corresponding stop got lost due to signal issues with Insteon).