Форумы / Cotonti / Core Labs / Ideas / CRON Engine

There are actually plugins that act like that. Mostly for Admin part, but they could use global hook instead. But there are a few pitfalls with this approach:
1. Performance loss due to overhead of calling task manager each request (at least a simple check, but still each request).
2. Interoperability and race conditions. This should be programmed in a "thead-safe" fashion.