The scenario you gave touches on a few issues.
1. If you were not originally scheduled to exceed 8 hours of flight time, then you would not need compensatory rest. The requirement for compensatory rest is if you are scheduled, not actually flown, more then 8 hours in a 24 hour period. If you actually flew more, because of delays due to wx, then no comp rest, but you must still have a legal rest period, which could be reducable.
2. The second issue, which is a biggie, is the 16 hour rule. At any point up until actual takeoff, you must be able to look back and find a legal rest period during the preceeding 24 hours. This comes from the now famous Whitlow interpretation, which has been upheld in court. If when you are about to takeoff, you know you will not get to your destination without exceeding the 16 hour rule, then you are illegal to go. If you exceed the 16 hour rule once airborne, ie ATC assigns an unexpected hold for an hour, then that would not be a violation of the rule because it was an unforeseen delay.
ALPA has a very good flight duty time interpretation paper on their web site,
www.alpa.org. Everyone should have a copy in their bag.