Thanks for the input guys, however I have been able to verify a couple of titles myself to close the loop on my initial query. I have been able to use a 'debug' feature in the Lumagen Radiance Pro that highlights on screen (with a red flashing area) any parts of the image that have an input nit level above a predetermined limit.
Above is a quick iPhone pic of Unchartered: Lost legacy.
The predetermined limit is set by the "Linear Y>" percentage - this is basically a percentage of the 10000nit HDR10 max (e.g. 10%=1000nits, 40%=4000nits, 100%=10000nits etc). So any bright red flashing elements seen on screen (such as the centre of the explosion in the above image) have an input nit level 'above' the "Linear Y>" setting that is currently active.
As you can see in this screen shot, there is content in the explosions above 9700nits input level, in fact the red flashing area only disappears at Linear Y=100%
Long and short of it then, and to answer my original query, it looks like HDR titles, at least on the PS4Pro, are essentially mastered using the entire HDR10 range and have highlights up to 10,000nits input.
I have also ran this check on Horizon: Zero Dawn, with the same result. I will check on Infamous First light/Second Son, however I expect the result will be the same.