Correct me if I'm wrong, but I think that method will work fine for the Original firmware and give the player a set of .pla files. You would then need to load up rockbox and convert the .pla file to a .m3u8 file. Correct?
I'm not working with WMP, so I don't know which format the playlist are, but I think that Rockbox got an intern playlist-converter.
And if that's not working, than you can use one of the programms for playlist converting in this forum.
If you use Media Monkeys, you don't have this problem and you can make much more settings for syncronisation.