There's 2 ways we can solve this:

(1) When loading a xptv format, don't create the SourceFactory ourselves, but make it go through the discoverer (which will create the thumbnails)
(2) Have the xptv format a directory format, in which we copy over the generated thumbnails.

Pros of (1):
* We make sure the sources are 'actually' usable on the machine (the project might have been created on another machine which has a plugin this machine doesn't have).

Cons of (1):
* Discovering takes time...

Pros of (2):
* We can store anything we want in it, not only limited to thumbnails
* We could have an option to move the source material to that directory

Cons of (2):
* If we switch to a directory system... we should decide *now* !

I'd be in favor of ... (1) + (2) in fact...

1) was implemented a while ago

Works as advertised (though very slow to open projects with lots of clips). Not sure if you have decided this method to be final?

*** Bug 589628 has been marked as a duplicate of this bug. ***