I asked Do any nonLDS christian denominations belief in baptism after death? but then realized that if the answer is no, it would be nearly impossible to answer, without going thourgh every possible Christian denomination. Is that okay?
I don't think such questions are really a problem. Sure, proving a negative is nearly impossible. However somebody with good domain expertise should be able to make an educated assertion when something doesn't exist. At the very least they would be able to guess the scale at which it might exist. For example the question about denominations would not be that difficult to answer. Even with some research you are only going to come up with a handful of individuals and with small followings outside of LDS circles that believe that. None of them have become denominations of any notable size. Thus it would not be too hard to answer in the negative.
The usual rules apply. Questions which are too broad in the positive are also too broad in the negative. But given a specific enough question that domain expertise and a little bit of research could speak to, I don't think the question formulation is problematic.
If you had phrased it like this:
"Which non-LDS Christian denominations believe in baptism after death(if any do)?"
Any answer would be limited to the affirmative case.