The best way depends..

If someone had written a plugin for ikiwiki, that would be the best way, but while it has one for flatter it doesn't have ones for these others.

If you want to put it on every page, edit page.tmpl and insert whatever html you like into there. This is also the best choice if the html includes javascript, <object>, or other stuff that ikiwiki's htmlscrubber strips out, since the admin of a site can bypass those protections by putting it into page.tmpl.

Or, you can add a template to your site, and put the stuff in there. Then use the template directive to include it into whatever pages you like. (Something like: [[!template id=bling]]) But on pages with the htmlscrubber enabled, the template content will be subjected to sanitization.

Is there a way to determine the canonical url of the page? BASEURL resolves to some like ../../.., URL is empty.