Crayola Boy,
OK... your example wasn't enough to give me the complete picture, so I did a bit of googling and found a nice explanation at
http://www.xml.com/pub/a/2004/11/03/itunes.html.
Basically, here's the explanation of what you need to generate:
<key>839</key>
<dict>
<key>Track ID</key><integer>839</integer>
<key>Name</key><string>Sweet Georgia Brown</string>
<key>Artist</key><string>Count Basie & His Orchestra</string>
<key>Composer</key><string>Bernie/Pinkard/Casey</string>
<key>Album</key><string>Prime Time</string>
<key>Genre</key><string>Jazz</string>
<key>Kind</key><string>Protected AAC audio file</string>
<key>Size</key><integer>3771502</integer>
<key>Total Time</key><integer>219173</integer>
<key>Disc Number</key><integer>1</integer>
<key>Disc Count</key><integer>1</integer>
<key>Track Number</key><integer>3</integer>
<key>Track Count</key><integer>8</integer>
<key>Year</key><integer>1977</integer>
<key>Date Modified</key><date>2004-06-16T18:10:55Z</date>
<key>Date Added</key><date>2004-06-16T18:08:31Z</date>
<key>Bit Rate</key><integer>128</integer>
<key>Sample Rate</key><integer>44100</integer>
<key>Play Count</key><integer>3</integer>
<key>Play Date</key><integer>-1119376103</integer>
<key>Play Date UTC</key><date>2004-08-17T16:39:53Z</date>
<key>Rating</key><integer>100</integer>
<key>Artwork Count</key><integer>1</integer>
<key>File Type</key><integer>1295274016</integer>
<key>File Creator</key><integer>1752133483</integer>
<key>Location</key><string>file://localhost/Users/niel/Music/ \
iTunes/iTunes%20Music/Count%20Basie%20&%20His%20Orchestra/ \
Prime%20Time/03%20Sweet%20Georgia%20Brown.m4p</string>
<key>File Folder Count</key><integer>4</integer>
<key>Library Folder Count</key><integer>1</integer>
</dict>
The collection of tracks is a dict, keyed by the track ID. Within each track, also a dict, the keys include Track ID, Name, Artist, Album, Genre, Kind, Size, Total Time, Disc Number, Disc Count, Track Number, Track Count, Year, Date Modified, Date Added, Bit Rate, Sample Rate, Play Count, Play Date, Play Date UTC, File Type, File Creator, Location, File Folder Count, and Library Folder Count. Although most of these are self-explanatory, a few bear further explanation.
Kind refers to the file encoding. Valid values include AAC audio file and MPEG audio file.
Play Date and Play Date UTC contain the date of the last time the track was played to the end, in local and UTC time, respectively.
File Type and File Creator contain the Mac-OS-specific file type and creator, a long integer that indicates the program that created the file, and a particular file type. These long integers are usually represented as a mnemonic four-character code.
Location contains the URL of the audio file, with a file URL scheme.
So this should be pretty straightforward, with perhaps the conversion of the dates and time.
I think I've got about a week of down time where I can spend a few hours on this (assuming I can VNC into my computer back home), but I'll need a few things:
1) Give me a complete example of an iTunes Music Library file with just a handful of media items and no playlists (I don't think you can get at the playlists with MCXMLExport).
2) Give me a complete example of an MPL (just export a playlist as MPL) with just a handful of media items in it.
3) Tell me which fields should line up.
Then I'll write some XSLT that goes from MPL to iTunes Music Library and you can see how that works. I'm unsure on how some of the fields that don't map will work for your purpose, but I'm assuming a default value will work just fine.