GPL is certainly not the only free license. And what about people that go the "GPL\0for files in the \"GPL\" directory" way?
Well for the latter, obviously we'd fix the bug that allows poison null bytes to break a string, since that's a pretty serious security vulnerability in a web browser.
For the former, all of the following are valid in both HTML 4.01 Strict and XHTML 1.0:
<link rel="copyright" href="http://www.gnu.org/licenses/gpl.html" />
<link rel="copyright" href="http://www.opensource.org/licenses/mit-license.php" />
<link rel="copyright" href="http://www.microsoft.com/opensource/licenses.mspx#Ms-PL" />
And all of the following work in any included ECMAScript file:
// License: http://www.gnu.org/licenses/gpl.html
// License: http://www.opensource.org/licenses/mit-license.php
// License: http://www.microsoft.com/opensource/licenses.mspx#Ms-PL
You certainly have the freedom to alter your user agent to require any set of licenses you're comfortable with.