hollowpoint... what's the rush?! a turn doesn't take that long, can't you wait til you finish the turn and are on the radial?
here is the definition of established:
Established - Definition and More from the Free Merriam-Webster Dictionary
In my mind it's quite simple: if you're on the radial, you're established... if you're not on the radial, you're not established.
I can't find you a reference for that but I searched my pdf version of the instrument procedures manual and every time it said 'established' it was quite self-explanatory. The only time I've seen a 'definition' is with regard to being established in the hold where the IPM states that you are established when you've crossed the beacon after going around once.
The IPM also states that for approaches, descent is not to be commenced until you're within 5 degrees of your track. I don't see it written anywhere for your scenario but I would assume that if you're within 5 degrees of your track you are 'established'.
This is the quote from the IPM: "Descent from procedure turn altitude to final approach fix altitude can only be made when the aircraft is on course inbound to the beacon (tolerance is ± 5°)."