To add a condition to a genpage event hook would mean that it would be executed for everyone...this actually IMHO does add up, and I'm not convinced that everybody would need this functionality, especially not blogs which already run at a performance barrier...
There is a s9y crontab plugin, did you check that one? Otherplugins can even hook into that. But IMHO this trackback scheduling someone would more likely search within the trackback plugin, not the crontab plugin?PS: Maybe there is even further use for such a cron-like system?
REgards,
Garvin