I can think of a couple of ways to do it. The most obvious is to draw
three background frames (i.e. no flow tag assigned) on the master page.
The top and bottom frames would be horizontal rectangles, and the side
frame would be a narrow, vertical rectangle. Type your Xs into them.
Then, anytime you assign that master page style to a body page, the rows
of Xs will be there on the page.
You could probably also use Silicon Prairie's wonderful AutoText plugin
to create your background text frames, then paste them in at will on a
body page with a couple of keystrokes.
You can also create a full page image in an external app like Illustrator, and import it into the custom L/R master pages. Turn off run-around. Send to back.
> ... assign that master page style to a body page ...
This is unstable if done manually, because the MP assignment doesn't stay with the content as entire pages of other content are added/deleted at previous pages.
It can be partially automated if the "WP" body pages have at least one unique paragraph format. Use Apply Master Pages (which, when run the first time, creates a Reference Page with an empty mapping table). Add your unique tag name and MP names to the table.