Actually it's even easier than I thought. No need to create an MCL file, just drop a shortcut to your batch file in the right directory and MCE will pick it up.
Note: I've not tested this myself, I'm just going off the following entry from the MCE2005 SDK:
Adding Windows-based Applications
You can add any Windows-based application to the Media Center More Programs page by placing a shortcut to it in the following folder:
C:\Documents and Settings\User folder\Start Menu\Programs
\Accessories\Media Center\Media Center Programs
In this path, "User folder" can be either the All Users folder or the folder of the current user, whichever is appropriate. The name of the shortcut should be the name of your application, with no "shortcut_to_" prefix, and with the extension .lnk; for example, MyApplication.lnk. The name of the shortcut, minus the .lnk extension, is displayed as the button label on the Media Center More Programs page.
Adding an application to the Media Center More Programs page also causes it to be added to the Media Center Start page menu.
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