Thanks for the comments. Then it seems this functionality should go into main pd-x GUI. I will try to work it out - I'll just have to make the dialog window prettier because such an ugly window that was acceptable for the plugin cannot go into main pd :o)

What do you think about changing the pd_guiprefs interface in way like this:
 (current -> proposed:)
 domain (always "pd-extended") -> could be hardwired to "pd-extended"
 key (name of the owner module, eg. "recentfiles") -> domain
 (currently doesn't exist) -> key (so that you can target a single line in a config file)
 value (the content of the whole file) -> value (the value of a single key)
This change would make pd_guiprefs compatible with the .pdextended file as well. Of course, then recentfiles.conf will look something like this:
 recentfile1: /path/to/recentfile.pd
 recentfile2: /path/to/another.pd
 (etc.)

András