Hi Robin,
first I want to say that you're working on a really often-requested feature (it's filed as https://bugzilla.gnome.org/show_bug.cgi?id=415360 )
It would be awesome to have it.

I want to give my quick thoughts as a non-developer (as a tester, UI designer/usability freak, etc.). My understanding of the implementation is limited, but on the user side of things, wouldn't it be nicer if we instead had a workflow like the following?
1. Import your images in pitivi.
2. Select them in the source list.
3. Use a special "Project > Make stop-motion clip..." menu item (sensitive only if a bunch of still images are selected). This would show a dialog asking to specify the framerate or duration, and then it would create some kind of "meta-clip".
4. You drag that "meta-clip" in the timeline just like a normal clip.

On the UI side, this is much more efficient and it would be *a LOT* better in terms of UI performance (one cairo/goocanvas/thingy object instead of 27000 objects), and would be much more manageable (what if you want to change your settings along the way, or trim the whole thing, or move the whole thing to a different layer? Not very efficient if it's a bunch of 15ms clips).