"After three weeks," Sirius says carefully, now that Q has shifted to specifics rather than hypotheticals, "we will know for certain whether or not someone's possesed." He suspects that Severus' not asking any questions is a Bad Sign, but isn't about to bug him about it now, when there's a James!ghost in his kitchen.
"As for who can do it... St. Mungo's, obviously -- I'm sure they'd have someone who's capable. But they also have paperwork and the associated complications." Such as wanting real names and magical signatures and eighteen vital samples before agreeing to a(n elective) consultation and so on. "My advice would be to see if you can find someone freelance, or who has his or her own practice, who has an interest in this#. You could even approach them as a researcher, rather than as a patient, and learn the technique for yourself.##"
I'm all for faking it :D
But I think there's likely to be a special way of developing the picture as well as needing special light when it's taken.
# Can be Sirius. But see below, also!
## Given the limits of fonts and so on, please consider this option to be jumping up and down squealing 'pick me! pick me!' because progressing the soul imaging won't be held up by my/Sirius' availability/knowledge :D