It could probably be done with scripting, then assigning a shortcut to the script.
I am afraid I am not smart enough to do any scripting. Oh, how I wish I could, though!
Only programmers can make scripts, correct?
If you ask in the InDesign Scripting forum (or perhaps here), someone might make a script or point you to a script.
> Only programmers can make scripts, correct?
Only if you think you can't. It takes more determination than smarts :-)
Here is one, written from memory. Fill in the right name for the layer you want to toggle, and save in your User Scripts folder under a suitable name (whatever you use, make sure it ends with .jsx). It will appear in your Scripts panel right away and you can test if it works by double-clicking. If it works, you can assign a shortcut key for it using the Edit Keyboard Shortcuts menu item -- look for it in the section "Scripts".
app.activeDocument.layers.item("YourLayerName").visible ^= true;
A line-by-line description of what it does: it toggles the "visibility" state of the layer called "YourLayerName" in the active document!
I wish I could do that, but it just seems to hard to me. I am sorry I am not smarter at these things.
Is there some reason you aren't using ruler guides?
You know what? I'm an idiot. I should have been using the ruler guides all along.
(Sigh) ... so much to learn.
The ruler guides are the things you just "yank down" from the ruler with your mouse, correct? They look light blue on my screen.