OP here. I just came by the announcement of the Content Experiment API [1] through Analytics' newsletter (and I'm not the only one around here), and I'm very excited because my plugin appears to be complementary with it. Support for this API pushed up in the priority queue and will be available soon. In the meantime, use the good old custom variables and events to track your changes.
[1] http://analytics.blogspot.fr/2013/06/google-analytics-become...