I've got an effect on a bus that I need to automate via MIDI, but I can't figure out how - or if there even is a way - to do that. Any ideas?

It should not matter if the effect is on a track or a bus. It requires a MIDI track to handle routing the incoming MIDI and the effect setup correctly to receive the MIDI data from the MIDI track
 
Prior to X3, in order for an effect to accept MIDI input it has to be configured as a synth in the Plug-in Properties accessible in the drop down in the VST header or Plug-In Manager. This has the side effect of moving the plug-in from the effect list to the synth list.
 
Starting with X3 any effect capable of MIDI I/O can have the feature enabled by the drop down in the VST header.
 
 

Thanks, Steve. It appears to be an issue with this particular plugin, I guess, that it's not responding to MIDI input. I'll take it up with the vendor.

In X3 (not sure if it exists in older versions) there is an entry for each effect in the VST Inventory called WantEvents. This is set to 1 if the VST scanner determines the plug-in can accept MIDI input.

Which plugin is it?

This is MDynamicEQ. I'm at 8.5 so I have to use the "configure as synth" option. WantEvents is set to true whether configured as a synth or not. It appears the only thing that changes with the synth option is the IsSynth key.
 
The problem is already moot, though. I just wanted to suppress the dynamic gain parameter for a low-shelf filter during an a capella vocal intro because it was sounding too thin when naked but needed some high-pass filtering when the instruments kick in. The easy solution was to simply move the vocal clip into another track and EQ it separately. 
 
I'm very high on this plugin, but disappointed that there appears to be no support for automating this parameter except via MIDI. It's the first negative I've encountered with it.

I've been spending quite some time with the Melda plugins. A suite of plugins hasn't had me this excited before. But I hadn't encountered your specific use issue.

bitflipper
This is MDynamicEQ. I'm at 8.5 so I have to use the "configure as synth" option. WantEvents is set to true whether configured as a synth or not. It appears the only thing that changes with the synth option is the IsSynth key.
 

That all sounds right to me. AFAIK WantEvents is set based on the plug-in's capabilities determined by the scanner. The MDynamicEQ can accept MIDI input. But in SONAR 8.5 a plug-in will not be sent MIDI data unless it is configured as a synth, which updates the IsSynth key.