Same thing would go for music hopefully "play some rock"
Assuming your music is reasonably tagged, you can create this command quite easily by cloning a similar command and changing the parameter to genreid rather than, say artistid, and then pointing to the genres payload xml file.
To figure this out for you, I started with the "Play album" command in the standard XBMC configuration, and went from there:
<?xml version="1.0" encoding="utf-16"?>
<!--VoxCommando 1.9.3.3-->
<command id="486" name="Play Genre {1}" enabled="true" alwaysOn="False" confirm="False" requiredConfidence="0" loop="False" loopDelay="0" loopMax="0" description="">
<action>
<cmdType>XJson.Raw</cmdType>
<params>
<param>Playlist.Clear</param>
<param>"playlistid": 0</param>
</params>
<cmdRepeat>1</cmdRepeat>
</action>
<action>
<cmdType>XJson.Raw</cmdType>
<params>
<param>Playlist.Add</param>
<param>"playlistid": 0, "item": {"genreid": {1}}</param>
</params>
<cmdRepeat>1</cmdRepeat>
</action>
<action>
<cmdType>XJson.Raw</cmdType>
<params>
<param>Player.Open</param>
<param>"item": {"playlistid": 0}</param>
</params>
<cmdRepeat>1</cmdRepeat>
</action>
<action>
<cmdType>XJson.Raw</cmdType>
<params>
<param>GUI.ActivateWindow</param>
<param>"window": "musicplaylist"</param>
</params>
<cmdRepeat>1</cmdRepeat>
</action>
<phrase>Play</phrase>
<phrase>genre, me some</phrase>
<payloadFromXML phraseOnly="False" use2partPhrase="False" phraseConnector="by" Phrase2wildcard="anyone" optional="False">XbmcPayloads\xbmcGenres.xml</payloadFromXML>
</command>
... The only media I have is music, so I can't help with the movie genres, but perhaps you can try a similar problem-solving approach based on the existing commands and the XBMC documentation (
http://wiki.xbmc.org/index.php?title=JSON-RPC_API/v6).
If you generate your XBMC payload xml using the Xsql plugin, you likely already have a xbmcVideoGenres.xml file in your XbmcPayloads folder.