Maybe this will help.
D1.5 seems to treat stress relieving differently than PWHT, although Sec 12.15.2 tells us any heating above 900deg F by sources other than welding (with some exceptions) is considered PWHT. At least for Fracture Critical concerns.

The latter part of Sec 12.15.2.5 says that MT or UT when required must be either repeated or performed after everything cools to ambient temps.

12.15.2.2 says the weldment shall not be allowed to cool below the minimum preheat or interpass temp before being raised to PWHT temp.

Section 9.21.5.1 says testing may begin after welds have cooled to ambient temps.

Since you stated PWHT instead of Stress Relief Heat Treatment, I assume you might be dealing with FCP weldments.

So, if Section 12 is applicable in your situation, you're not supposed to cool the weldment enough to be able to do NDT before you begin PWHT. And, if NDT had been done, it would be the same as "informational-only" because you'll have to do it again for acceptance.

There are still a few holes in all of this in that RT is not mentioned in 12.15.2.5, only MT and UT. Also Section 4.4.2.3 and Table 4.6 indicate it is possible to stress relieve at 900 deg F which may or may not be considered PWHT but would be thermal stress relieving (I'm still fuzzy on what the real difference is, they seem the same to me).

But, 12.15.2.1 says the Engineer's approval is required. So that could be your saving clause or your dissapointment, depending on the answer.

Now that I've thoroughly confused myself at this late hour, all I have left to say is good luck.
CHGuilford