That is a very cool system, and it looks great for creating new applications in no time (I might use it for that kind of thing in the future).
Zenphoto is designed to be simple from the ground up, and I think it'll stay that way. It uses its own database persistence class now (in SVN) and only includes the code it needs to run, and each part is highly optimized for its specific purpose (especially things like the image generator, the lazy filesystem traversals, the database usage) so to keep everything simple and fast and flexible as possible, I prefer not to have code generated for me ;-)
Though you can imagine how much faster I'd be! :-P
I agree also that requiring PHP5 is out of the question today.