This might not be too difficult, I've never looked at the playlist file once it is on the iPod, but the ones you can save are pretty simple XMLs. It might be an easy translation.
I've written a piece of code using the iTunes COM API that will allows you to create /copy a playlist, created in MP, in iTunes as a UserPlaylist. There is an option in the interface to create the playlist in a source also ( ipod can be a source) and then update the ipod. I haven't tryied that yet, as I don't have a working ipod, mine died.
Code needs a bit of tidying up to release as a plugin, but if you interested intesting PM me.
I am currently working with an little tool called iTunesExportUI. This exports the playlists you want to a directory. Working fine with MP. The only little thing is, that for some reasons i dont know MP is not reading the tags properly although they are correctly in Database.
But i think this issue will go away with SteveV's new tag reading system. I made a little batch file which I click for once a week or so and so i have always the newest playlists on my HTPC.
You find more informations here: http://www.ericdaugherty.com/dev/itunesexport
I've written a quick.. and i mean quick.. windows plugin that lists all the current playlists in MP and exports them to an iPod though the iTunes API.
I've tested it on adding playlists to iTunes and it work fine, but I haven't tested updating an iPod yet as mine's bust, so anyone who wants to test.. I'll post a beta in the Plugin Dev forum later tonight. N.B. It's basic hack together at the moment.. no error handleing or such, first time writing a windows plugin as well so go gentle on me.. MPs log does catch any exceptions though so post me a log if it doesn't work...
You just need to click the playlist you want to add to your iPod... simple... I hope..