I don't think you can. Some of the variables accept arguments (like date formatting) but <%Canvas%> doesn't seem to. I could be missing something, I never tried it before I saw your question :)
Workarounds if you really only want to show the same text on several different canvasses without having to edit it everywhere separately (which seems to be what the original question you linked is about):
1.) Standard or Pro: Put the text you want to show in one of the info fields like Subject or Description in the Document: Data inspector, and use <%Subject%> or <%Description%> to display it etc.
2.) Pro only: Use a shared layer with the text, and apply it to all canvasses where it should show.