Cannot populate settings subform for custom source

I'm creating a custom source module for filefield_sources to allow grabbing files from some custom file server application.

As I need to provide some settings for the source on each field settings (a server uri and some credentials), I would like to populate a source settings subform that would display into the "File Sources" fieldset of the field form, just like any other included file sources (eg: autocomplete reference options), but I can't figure out how to achieve this.

As far as I understand, source settings subform are included in the field settings form thru filefield_sources_form($instance, &$form_state), which gets sources subforms thru filefield_sources_invoke_all('settings', $params).

Pb is that filefield_sources_invoke_all('settings', $params) gets its sources exclusively from filefield_sources_includes(), which fetches sources that only reside in filefield_sources/sources directory...

Am I mistaking something ?

Thanks for your help and ideas