To update, I got a chance to check this out and no, the .m3u format playlists created (and show and play correctly in the OF) do not
show in the playlist catalogue in Rockbox.
In consulting the Rockbox manual, I find:
Note: All playlists in the Playlist catalogue are stored by default in the /Playlists directory in the root of your player’s disk and playlists stored in other locations are not included in the catalogue. It is however possible to move existing playlists there (see section 4.1.2).
I know in the OF, .m3u playlists should be placed in the Music folder (alongside the folders/files contained in the playlist). Apparently this is different in Rockbox and must be placed in the /Playlists folder.
Haven't had a chance to check this out yet myself, but maybe you could give it a try and post back with your results.