Data liberation is different from open-sourcing. Delicious allows you to dump every bit of metadata about your bookmarks either through their Data Export menu or via their API. Yahoo! did fine with Delicious in terms of data liberation. I imported all my bookmarks into pinboard.in without any trouble at all.
Open sourcing it wouldn't solve the discovery/hosting part, but I still think they should do it.
I'm sure it wouldn't be "easy," but it wouldn't be unprecedented.
Yahoo! folks also should take a look at dataliberation.org, for best practices on getting on data out (also a Google joint).
(Nope, I don't work for Google :D, just think they've done some good things in this space. )