If you create a Page Rule that does a 302 redirect to something with ð in the name then save (either as draft or deploy the page rule will get rewritten to ð. If you try to navigate to the page (such that you are redirected) you are in fact redirected to a url with ð in it rather than ð in it.
For example, a page rule to redirect to http://foo.bar.com/?barðanol will redirect to http://foo.bar.com/?barðanol. Other characters after the ampersand don’t seem to matter.
Hmm, I just tested again and this time it did go through. I do know that originally I tested and was getting the ð in the browser. I’m uncertain what changed (I fiddled with a bunch to try to fix it, so maybe I accidentally fixed it).