Basically you'd need to integrate Boxee as a DCE wrapped media player under LinuxMCE, which would mean, you need:
(1) a way to send commands to Boxee.
(2) Boxee needs to be able to run well in windowed mode, with the ability to stretch to any window size, as Orbiter needs to manage windows itself.
(3) write the plugin on the core to create the streams to pass to...
(4) the Player device that spawns Boxee, and provides the communication channel so that DCE commands can be translated to actions in Boxee, as well as providing the event channel so we know when things change in Boxee's state.
(5) The plugin also provides data grid generators which can be used to fill in various screens that can be displayed on Orbiters This is important, because it provides a dimension of UI interaction that is NOT available without LinuxMCE.
With those things above, a Boxee Plugin and a Boxee Player could be made to integrate Boxee into the system. This isn't difficult, and any intermediate developer can do these things in a matter of hours provided Boxee behaves well enough to work in this manner.
Who wants to take it up?