Unfortunately it's a lot harder to get secrets out of a priest than a wizard, so I'd expect we'd have have to do a really big favour for them or tempt them to join the project via some solid early successes.
Why, actually? Are priests really under stronger secrecy obligations than wizards, or is it just that the priestly trust in their god is antithetical to controlled experiments?
Would explanations come easier to a priest of Hekarti, for example, or another elven god? Mathilde's in Laurelorn, she can interview plenty of those.