But then I guess I cannot use ENTER (numeric keypad) because a AutoHotKey would be "global" and "on all the time".
DOH!
 
(Maybe I should just tap my rewind after stop and succumb to that - even if not the ideal workflow I had in mind)
 

If you want a single key binding for the two operations in SONAR, you need some tool like AutoHotKey. AutoHotKey installs a default ahk script in the root of your "My Documents" directory. I have AutoHotKey setup to run at start up. When running. its icon resides in the notification area. Right-click on the "H" icon and "Edit This Script."" This will open the default script in notepad. Add the ahk script above and save the file then right-click on the "H" icon again "Reload this script." The binding is global and there are ways to guard it from firing outside of SONAR but it is unlikely you will be using the keybinding without SONAR running. I do not use a SONAR key binding in AutoHotKey. It is just as fast to run the CAL script from the SONAR menu and CAL open dialog.
 
The SONAR only solution would be to use two key bindings. One to stop the transport at Now and another to run the back2 CAL script.

Or if you are lazy like I am, you can spring for a programmable keyboard that allows you to record macros galore. I recently purchased one: http://www.amazon.com/Razer-BlackWidow-Chroma-Mechanical-Keyboard/dp/B00MTWV0II/ref=sr_1_2?ie=UTF8&qid=1439610200&sr=8-2&keywords=razer+blackwidow .
 
The keys are "clicky" which might not suit some. Other brands are quieter. This keyboard also lets me highlight individual keys in different colors, which helps me quickly see my common hotkeys. I set macros to go to the next / previous marker - 1 measure so I can pick up any keyswitches for the section.