Plugin to make spent time required when an issue is closed

Hi,

Im playing with the plugin architecture and came across this question: if i want to make spen time required when an issue is closed
If i:

wrap the issue's before_save method i dont have the time entry to validate it
wrap the controllers edit_before_save action, how can i return immediatly after i detect that there is a state change without a time entry?

Thanx!