It would seem to me that if it has holes in it (ports) then it should be possible to pull a vacuum (maybe using a venturi effect). Anyway, if I was in your shoes I would approach the client for their opinion on the matter. By the tone of your post, however, I would guess that you've already done that.
I would suggest that since you used ASME (I would guess Section VIII) as a guideline for fabrication, you propose ASME VIII also for inspection. UW-50 might provide some guidance here - you would only have to look for cracks (but it requires 100% examination of the listed welds). If that doesn't fly UW-52 would seem to be appropriate based on your statement that "customer is requiring UT of certain welds" - that could be interpreted as spot examination.
Mankenberg