I would like a way, if one does not currently exist, to access MIDI plugins, the arpeggiator, and other MIDI settings in an instrument track. It's really annoying to have to split the instrument track every time you want to, for example, use the arpeggiator. 

Your wish is granted!  Scroll down to Figure 176 here:
https://www.cakewalk.com/Documentation?product=SONAR%20X2&language=3&help=Inspectors.3.html
 

Good suggestion, Jeremy, and thanks for the reminder, SteveC, but the inspector controls only I think Jeremy's original idea might be better envisioned as a bank of controls within Sonar that could be mapped to the VST parameters. 
 
Picture it as generic controls (similar to those on the Instrument page of Rapture Pro) that can appear on the main screen like the Inspector Panels, but that can be mapped to whatever elements in the track plugins (instruments OR FX) that you want to control while working in track view.  
 
This could really improve workflow and might help to make Sonar useful for live performance.  It would simplify mapping to hardware, too, if you could map these generic controls in Sonar to physical controls.  
 
Lets see if we can get some support for this idea!