The only time you can stay longer in that hold & use yr Alt reserves is when you know from a legal standpoint & a common sense standpoint for that matter is that the wx at yr original destination is now at or above yr Ldg's req's now known from say an updated TTF.
That is my understanding as well Wally.
If in my scenario in the first post the weather remains BKN at 600ft then I bug out after ten minutes to YSCB. If however, a TTF is issued before then stating the cloud BKN at 800ft I can remain in the hold and continue to YSSY as it does not require an alternate.
I know all this is a bit fanciful as I am trying to avoid getting into the airmanship and decision making side of things and isolating it strictly to legal requirements.
I know of a few pilots who would 'commit' to YSSY in the scenario I gave because they are confident that they'll get in. As I understand it, and until I can find some reference to 'committing' , they are then operating without the legal minimum of fuel onboard.