At the very least, if a plugin has a hidden custom parameter (to which data is stored when another parameter is manipulated), will these at least come over into FCPX and work there if we don't explicitly "export" them? Yes. All your parameters are still there, they just don't all show up for the user. You will still get -parameterChanged: notifications for them and you can change them using the various parameter APIs.
At the very least, if a plugin has a hidden custom parameter (to which data is stored when another parameter is manipulated), will these at least come over into FCPX and work there if we don't explicitly "export" them?