I just came across this question and saw that this was impossible, but stubbornly kept trying anyway. Good news, because this actually can be done with relative links!
If you look at the documentation for relative links on GitHub, you can see that it supports ./ and ../ operands.
Given that your README is located at:
https://github.com/<user-name>/<repo-name>/blob/master/README.md
… and the wiki is located at:
https://github.com/<user-name>/<repo-name>/wiki
… that means you can just crawl back from the README to the wiki, like so:
[My Awesome Wiki](../../wiki)
Update: As some people have pointed out in the comments (thanks!), please be aware that these relative links will break on clones and forks, so be sure to weigh that into your decision!