This isn't the answer you are looking for, but maybe I can help a little.
If you look in the keyboard shortcuts and search for "playback" the one under "edit" will at least allow you to call up the window faster than using menus. I assigned Ctrl-Shift-P because I don't care about setting poster frames and I can remember the "P" for "playback".
I don't see any way to select or deselect the monitor via the keyboard. At least not from within Premiere Pro. If you were a moderately talented programmer, there are ways to control Premiere Pro from outside of the program itself. I used to write programs to control other programs but I haven't done so since about 1994.
Perhaps submitting a feature request would be appropriate. https://www.adobe.com/cfusion/mmform/index.cfm?name=wishform
The Program Monitor has to be 'selected' (yellow outline) when you hit the CTRL+` combo. This will maximize the playback on your primary GUI display.
Thanks Steven and Jim, I appreciate both your answers. What I've I've since discovered is if I keep the Premier Pro Reference Monitor up on my left hand monitor that I'm not editing on, whenever I want to go full screen on that monitor all I have to do, as Jim says, is highlight it then press F2 (which I programmed for full screen) and bob's your uncle. Not quite as seemless as Media Composer's toggle button, but a work-around!
Many thanks guys for your time,
On my plain vanilla install (Window) it's Ctrl+' which works perfectly. I had the same issue going into the playback menu and select a monitor which was quite tedious.