I don't think it responds to MIDI. Here's what the help file has to say:
The X-Ray Windows feature eliminates the need to constantly minimize, move, or close windows in order to work in other windows. It works by decreasing the opacity of the current window enough so that you can see and work with the window that's behind the current window. You activate the feature by pressing a keyboard shortcut (default shortcut is Shift+X) when the mouse cursor is over a window you want to x-ray. You can choose to X-Ray whichever window is underneath the mouse cursor, or automatically X-Ray all FX/synth property pages in one step (note: the mouse cursor does not need to be over any plug-in property pages).
The X-Ray Windows feature works on the following windows:
Piano Roll view (when float-enabled)
Snap To Grid dialog
Plug-in effects and synths
To Select Key Bindings for X-Ray Windows
Use the Options-Key Bindings command to open the Key Bindings dialog.
If you want to use currently unassigned keys or key combinations, scroll through the options in the Key window until the Global Key Assignment field that is just under the window reads Unassigned. It's a good idea to find two unassigned options that are next to each other or easy to remember.
Note: for best results with X-Ray Windows, avoid using Alt key combinations.
Once you've decided on two keys or key combinations that you want to use, select Global Bindings in the Bind Context field, and scroll to the bottom of the list of commands that are in the window below that field.
In the Key window, highlight the key or key combination that you want to use for the X-Ray command, then highlight X-Ray in the function column of the list of commands, then click the Bind button to bind them together.
Now highlight the key or key combination that you want to use for the X-Ray All FX/Synths command, then Highlight X-Ray All FX/Synths in the function column of the list of commands, then click the Bind button to bind them together.
Click OK to close the dialog.
To Use X-Ray Windows
Use the Options-Global command to open the Global Options dialog, and on the General tab, make sure that the Enable X-Ray checkbox is enabled.
Make sure that the view windows you want to X-Ray are in the Floating-enabled state: to check this, click the view or fx icon that's in the upper left corner of a window, and select Enable Floating from the dropdown menu. If Disable Floating is in the menu, then the Floating option is already enabled.
Note: all FX/Synth/Control surface property pages are float-enabled by default.
To X-Ray or un-X-Ray a single window, move the mouse cursor over the window, and press the keyboard shortcut (default is Shift+X) for the X-Ray command. The window does not need to have focus (does not need to be the highlighted window).
To X-Ray or un-X-Ray all plug-in windows at once, press the key binding for the X-Ray All FX/Synths command.
Note: if a window has focus, and the window's Give All Keystrokes To Plug-in button is enabled, X-Ray keyboard commands won't work. To Adjust X-Ray Windows Options
Use the Options-Global command to open the Global Options dialog.
On the General tab, you can adust these options:
Enable X-Ray--enable or disable this checkbox to turn the X-Ray Windows feature on or off.
Opacity--adjust this value by typing in a value, or by clicking and holding the + or - button to adjust the final opacity percentage value that an X-Rayed window reaches.
Fade Out Time--adjust this value by typing in a value, or by clicking and holding the + or - button to adjust the amount of time that an X-Rayed window takes to reach its final opacity percentage value.
Fade In Time--adjust this value by clicking and holding the + or - button to adjust the amount of time that an X-Rayed window takes to restore its original opacity.
Click OK to close the dialog and accept your changes.
To Exclude a Plug-in from X-Ray Capablity
Some plug-ins (very few) use DirectDraw to create their windows. These windows appear jittery when X-Rayed.
To exclude a plug-in from X-Ray Capablity, follow these steps:
Open the Cakewalk Plug-in Manager: use the Tools-Cakewalk Plug-in Manager command.
In the Plug-in Categories window, select the category that the plug-in you want to exclude is in.
In the Registered Plug-ins window, select the plug-in that you want to exclude.
If the plug-in is a DirectX effect or an MFX, write down (or select and copy) the CLSID value that's in the CLSID field at the bottom of the dialog.
If the plug-in is a VST or VSTi, write down the VST ID value that's in the VST ID field at the bottom of the dialog.
Close the Plug-in Manager dialog.
Open the xrayexclude.ini file that's in your SONAR program folder (use Notepad).
At the end of the file, find the [EffectProps View] section.
You will see entries such as the following: ; Waves SSL EQ Stereo
Exclude your plug-in by creating a blank line below the last entry in the [EffectProps View] section, and then typing:
;[name of your plug-in, but withour brackets]
XRayExclude[type the next available number in XRayExclude list, but without brackets]=[VST ID number, with no brackets, or CLSID number, with curly braces at start and finish]
For example, if the last entry in the [EffectProps View] section was: ; Waves SSL EQ Stereo
And you wanted to exclude the Cakewalk FxDelay from the X-Ray Windows feature, after creating a blank line you would type: ; Cakewalk FxDelay
If there was also a VST version of the Cakewalk FxDelay, you would add another line: XRayExclude14=[some VST ID number, with no brackets]
Save and close the xrayexclude.ini file, and restart SONAR to implement your changes.