I don't know why some oncos give salvage RT without any hormone treatment, all the data shows that RT works better with at least a few months of HT beforehand to weaken it. However, you are where you are and it seems that your OH has moved into the incurable camp but that does not mean that there isn't loads they can still do.
A PET scan at this PSA level might not show anything so don't build your hopes up that you are going to get answers, just in case it doesn't work out that way. On the other hand, if the scan shows one or two hot spots away from the prostate bed, you could ask the onco whether it is possible to have radiotherapy to the hot spots. If the cancer clusters are in the prostate bed, there will be no more RT and the conversation with the onco will be about the different kinds of hormones and whether or not early chemo might be worth trying; there is lots of recent research to suggest that chemo given at this stage can make the hormones more effective for a longer period of time.
When you see the onco for the scan results, I would be asking why hormones weren't given alongside the RT. If you are unsure about whether the onco is up to date with modern PCa treatment regimes, you can always ask for a second opinion from a different onco.
Where on earth do you live that doesn't have PET scanning?????