The following have contributed to this essay.
The issue here turns on whether or not the concepts of "free" and/or "libre" should be used in an external links policy.
One way of arguing it
The Wikipedia external links policy advises: "What should be linked: [...] Sites that contain neutral and accurate material that cannot be integrated into the Wikipedia article due to copyright issues..." w:Wikipedia:External_links#What_should_be_linked. The general idea here is that material published elsewhere under a libre licence can be incorporated into Wikimedia projects, so links to such material is superfluous. The quantity of material which can be incorporated this way will increase if the current compatibility negotiations over GFDL and CC-BY-SA succeed. Because "free content" can generally be incorporated into Wikimedia projects, the necessity of an external link only remains when a copyright difference exists.
This line of argument would lead us to favour external links to what some call "non-free" content, and to favour incorporation of "free" content rather than linking to it.
Additionally, one could argue that editors who are aware of "free/libre" issues should be encouraged to licence their contributions in a Wikimedia-compatible fashion (e.g. dual GFDL/CC-BY-SA licencing) and incorporate them into Wikiversity rather than placing the materials externally. The underlying principle here is to make Wikiversity a better place rather than to turn Wikiversity into a traffic redirection facility to educational sites whose editors feel they are even better places than Wikiversity.
A different way of arguing it
There is an argument which runs in the opposite direction, drawing on the Wikimedia mission statement: "The mission of the Wikimedia Foundation is to empower and engage people around the world to collect and develop educational content under a free licence or in the public domain, and to disseminate it effectively and globally."
One could argue that by extension this mission justifies a bias towards linking to other sites which share a similar mission.
It is recommended that an external links policy should make no reference to concepts such as "free" or "libre", and that no bias for or against sites containing or not containing "free" or "libre" content should be made. Firstly, as seen above, plausible arguments can be made for both a bias for and against such links - the logic is therefore inconclusive. Secondly, Wikimedia projects are primarily governed by a neutrality principle when it comes to content, and favouring/discriminating against external sites on the basis of their licencing policies might be seen as a breach of neutrality. As the Wikipedia external links policy already reflects, there is only one kind of licencing which is a criterion for removing links: that the externally linked site breaches someone's copyright (illegality).