I have just finished a bunch of additional checking into this issue, and it looks like the Rythmbox program itself is not setup to read shoutcast radio streams (whether the mplayer plugin is included or not). From doing a bit of google mining, it looks there are a few rhythmbox plugins that have been created specifically for adding this feature, but none of these plugins are available from FreeBSD ports at the current time.
Since the Rhythmbox port already has a build option to include specific plugins (such as the ITunes plugin), it might be a good idea to ask the port maintainer to add the option to build the shoutcast plugin as well.
Alternatively, you might be able to get the shoutcast plugin to work by adding it directly to the PBI's plugins folder (look at /usr/pbi/rhythmbox-[i386 or amd64]/lib/rhythmbox/plugins), although I have not tried that yet myself. Many of the plugins have compiled libraries in them (*.so), so if the shoutcast plugin also has a compiled library, it might need to be compiled on FreeBSD first.
~ Ken Moore ~