Yes, the plugins do not need to be released under the GPL. They are modular to the core Hyperic Server and are not required to be GPL'd. In fact, some of them have had some issues because the technologies they worked with may be under the Apache or BSD license. You are free to do what is appropriate for you.
If you do the contributor agreement, it allows us to possibly include your plugin into the general distribution which all users will get with their downloads. Otherwise, it will stay on the HyperFORGE and will need to be downloaded individually.
