I am still a little unclear of why you can't radiograph it, but I will approach it from the aspect of a golden weld.
I am not familiar with your material spec 700re, . I will assume there is a reason your cannot MT the material in question. (If you can it would be your better option)
In the states, a golden weld is considered one where it cannot otherwise be examined by volumetric method in the Oil and Gas world (RT, UT)
For those welds, each pass is individual examined as you've described. However; you've listed a temperature limitation that is usually not allowed for PT.
"The plate is at 100c due to attachments on the back of the repair with a CEV of 0.5"
That's 212F. Typically, penetrant is limited to 125-135f.
Therefore you will need a specially qualified PT procedure and materials.
To my knowledge, only Sherwin and Ardrox manufacture Hi temperature PT materials.
http://www.sherwininc.com/crossRef.htmlSherwin Penetrant K-017 and Ardox P303A
Those will have to be used with their matching cleaner and developer.
This procedure will have to be qualified at the maximum and minimum temperatures expected.
Suggest using Sherwin's KDS panels which are in compliance with ASTM E-1417.
At best this procedure will provide a sensitivity level 1.5 or so.
If memory serves me correct K-017 is only good up to 350F.
Hopes this helps,
Gerald