Yes, it would be a huge undertaking. I've already rewritten my own apps for offline maps, calendar, notes, and I plan to write contacts and iTunes too. When there's a standard API comparable to Apple Events/OSA within those core apps, I'd hope to have enough leverage to persuade other app developers to accept pull requests.
We already have D-Bus to cover sending events to applications, what's really missing is a standard way to enable scripting. I've had a similar idea myself but never got around to implementing it, but I think a standard D-Bus interface applications could implement to expose a scripting dictionary along with a Python/Ruby/Whatever library to implement a more OSA-like interaction would go a long way.
