Is there a Winamp function/plugin that will skip playback of songs that have rating lower than a set minimum of stars?
While there is no way to automatically skip the playback of those songs, there is a way to generate a smart play list to perform this action for you. To do it, right click "Local Media", and then click "Add Smart View...".
Go to Advanced Mode, and then enter:
rating >= 2
You can use the >= or > operator. The above will generate a dynamic playlist (updates each time you add/remove music from the Winamp Media Library), with only songs rated at least 2 or more.
Just create a playlist with your >= 2 rated songs duplicated a few times. Shuffle will be statistically more likely to encounter higher rated songs.