Since I was able to find a solution, I am answering the question myself.
Unlike what I have read elsewhere, it is indeed possible to place wp-content outside of the document root. Open basedir needs to be disabled, and the path to the new folder needs to have sufficient permissions.
Concerning the issue with custom fonts, I found the solution here:
http://davidwalsh.name/cdn-fonts
https://serverfault.com/questions/162429/how-do-i-add-access-control-allow-origin-in-nginx
So yes, it is possible to use them from a different domain.
The only issue that possibly remains is plugins updating the database, but in the rare event that this happens, I am assuming it can be fixed by disabling/enabling the plugin on the respective sites.