...appears to be impossible.  I've searched around here and couldn't find anyone knowing how to do it.
 
I have a VSTi (RealGuitar) that I have some "tricks" it performs (slides, hammer ons), depending on velocity (MIDI "Gain", as the knob says in SONAR), and I wanted to put in some random velocity increases QUICKLY and "live", rather than drawing things in.  (Actually, I ended up going to Event View and typing in values instead--seemed faster than drawing).  Right-clicking on the "Gain" knob on a MIDI track has automation recording grayed out.
 
Anyone ever find a workaround?

Kind of....
Not in the traditional sense of recording automation into an automation lane but the velocity offset may be played live and the audio recorded. Bind the velocity offset to a controller using Remote Control. Using soft synth audio recording capture the audio while playing the MIDI track.

You can't automate the knob, but you can create and envelope to effectively automate velocity offset via Clip Automation > Velocity in the track's Edit Filter.