since I couldn't find anything about it I wrote a template to create packer plugins with PowerBasic (http://www.powerbasic.com)
you can find the template here, call it a wcxhead.bas

it's actually a very simple sample as well that extracts all functions and subs from the source and presents them as files, nothing fancy.
I'll post a full fledged plugin w/ source once I figure a good archive format that hasn't been done before, with the ability to pack/update, etc. for study purposes as well.
Same goes for Lister and Filesystem...
it's public domain, so if you find it of use, use it....
I would appreciate it if you let me know if you use it for a plugin (email in source).
Christian Ghisler is of course welcome to add it to the SDK as another means of creating plugins.
Cheers,
Jeroen.