There isn't a way to do this directly in the plugin.xml file currently, but I think it'd be do-able if were to integrate Groovy scripting. There's been alot of Groovy integration happening on the server-side, very cool stuff, but that's another topic. In the meantime, you'd either need to write a Java class or scripting language of choice to collect the data and calculate the percentage. With the Java option, you'd need to compile the class and package a plugin.jar. With the scripting option, the plugin.xml can reference an external script or have it embedded within the .xml, but either way it runs out-of-process and would require, with Perl for example, an SNMP module to be installed. The Groovy idea would be best of both worlds, in-process scripting in the plugin.xml with access to the bundled support classes.
Let me know what your preference is and I can provide further direction.