I really don't understand how would be possible create plugin for gimp using paint.net.
But you are talking about Filter Factory filter and derivates, Gimp has already a a plugin to read,edit and use ffl, afs and similar.
It is called UserFilter, i mirror the Windows version renamed FilterFactory converter here
http://browse.deviantart.com/?qh=§i ... y#/d1on0rmand you may want download that, even if you use Linux because the zip include a few thousends of filter already converted (the UserFilter use internally a language somehow close to that of Filter Meister but convertion or as to be done one by one, or in batch using a utility included, but not directly accessible from the plugin interface )