I got to use Real Alternative's MPC by doing the following modifications on the playercorefactory.xml file (located at the Boxee's System directory):

Added this at the beginning of the file (after <playercorefactory>):

<players>
<player name="MPC-HC" type="ExternalPlayer" audio="true" video="true">
<filename>C:\Program Files\Real Alternative\Media Player Classic\mplayerc.exe</filename>
<args>"{1}" /fullscreen /open </args>
<hidexbmc>false</hidexbmc>
<hideconsole>false</hideconsole>
<warpcursor>none</warpcursor>
</player>
</players>

And added this after </players>:

<rules action="prepend">
<rule filetypes="rmvb" player="MPC-HC" />
</rules>

That made my Real Media files open (not play) in full screen with MPC.