Is it okay to ask a senior academic a question that my advisor is unable to answer?
At the heart of this is a interpersonal social question. When a student comes to me with a question I don't know the answer to, I rarely just say sorry I don't know. As an advisor, our goal is to help students learn what they need. When I don't know something, I try and point my students in a direction (textbook, articles, person) where they might be able to find the answer. While I would not be upset if a student "bypassed" me to talk to someone after finding out I did not know the answer, I can imagine some advisors would be.

I suggest you identify the researcher(s) who might know the answer and tell your advisor that you were thinking of contacting this individual. Your advisor might have some valuable insight and tell you not to contact Dr Jones, but instead contact Dr Smith. Your advisor might also give you an introduction or want to be included in any email correspondence so they can learn the answer too. Your advisor might simply say sounds good and leave you to it.