AFAIK there is no MCWS, core command or [edit] Media Center [/edit] command line option to eject a disc from a cd/dvd/bluray disc drive.
There are command line options to playback an audio/dvd/bluray disc, but they seem to be for Windows only as the drive id can only be specified as a drive letter e.g. MC32.exe /AudioCD D: according to the Wiki.
So, AFAIK, I don't think using a linux headless MC server as a 'CD/DVD/bluray player' is currently possible via scripting/JRemote(2)/Panel/MC client.
Searching the forum, it doesn't seem to be a much-requested feature either, most MC users seem to prefer the 'rip, import, then play from library' model.
I personally think it would be useful to have the capability cross-platform to be able to rip and playback CD/DVD/bluray discs on headless MC servers using JRemote or Panel.
I'm still happy to push the eject button on my server optical drive to get the disc back so not too concerned about an eject command.