Assuming they ever wanted the job and were the most desirable candidate—which those assumptions are probably not ever going to be true. But let's just pretend ... could they be hired as BYU head coach?
I think it would be difficult and would only happen if they were clearly better than any LDS option available. Which would be sad, because that would mean we'd have very few LDS coaches being successful anywhere. But in that situation, maybe the board of trustees could be persuaded only because they both spent significant time at BYU and would know what they were getting into and would know the importance of following BYU/LDS faith guidelines in mentoring young men.
I guess my point is BYU would never roll the dice with an unknown coach in those high profile positions who wasn't LDS. But a known entity like Grimes ... assuming he engendered trust while here ... maybe.
But the situation would be so exceptional that for all purposes is not ever going to happen.