Updated 02/25/2019: Added "LMB-RMB-Enter" and "LMB-MMB-Escape" as separate utilities to send an Enter (green check) or an Escape (red X) by mouse only.
With these, the green check and the red X don't have to be in the S menu or gestures (but can be if you wish).
Press and hold the left mouse button (LMB) and then click the right mouse (RMB) button for an Enter.
Press and hold the left mouse button (LMB) and then click the middle mouse button (MMB) for an Escape.
Since you're left clicking for either of these, be sure your cursor is not on something that will derail what you're attempting to do.
These are the icons that will appear in the lower right corner of your screen (Suspend, Exit and Pause are as outlined below):
The letters are a reminder of the buttons, but they won't be this legible in the lower corner of your TV.
Attached is a utility that maps the middle mouse button together with the right mouse button to display the shortcut "S" menu (for the current context, as usual).
Press and hold the MMMB then press RMB and the S meu displays.
By adding the green check OK and the red X cancel to each of your S menus and by using this utitlity, (3) gesture slots are freed up.
Although this utility is intended to make room on the gesture wheel, it can be used regardless of whether you use gestures or not.
While running, this utility is not restricted to SolidWorks, but makes little sense to use it outside of SW.
Right-click the "MR S" (middle button-rightbutton-S menu) in the lower right of your screen:
Select "Suspend Hoykeys" to turn off this button combination without exiting the utility.
Select "Exit" to terminate the utility.
Selecting pause doesn't do anything for this utility.
To install, just copy the attached file to a folder of your choice and double-click the exe to run it.
No modifications to Window Registry, or any other such things, are made.
To uninstall, just delete the file.
Questions and comments are welcome.