There are meaningful ways of answering such questions, more than just saying yes/no:
Absence of evidence
From Negative evidence on Rational Wiki:
Negative evidence occurs when an absence of evidence is actively found when actively looking for something. (Don't see your car in the garage? It's probably not in the garage.) An appeal to ignorance occurs when an absence of evidence is found when nobody has done any looking! (Maybe you should actually walk to the garage and check, before assuming your car isn't there.)
Thus, to find negative evidence, we can (a) identify a likely consequence (e.g., there would exist a scholarly fatwa claiming that XYZ is haram), and (b) find no evidence of the consequence, despite efforts to find such evidence.
In these circumstances, we can perform a literature search (e.g. searching various fatwa sites) and document our efforts. This allows the reader to distinguish between "we did not observe this, because we did not try" and "we did not observe this, despite our efforts".
Absence of evidence, or the failure to observe evidence that favors a hypothesis, is evidence against that hypothesis. This is because we are significantly more likely not to see evidence for a hypothesis when it is false than not to see it when it's true ... (Rational Wiki)
Example: Here's an example from travel.SE of how to apply an "absence of evidence" argument:
Question: Can I see the American Airlines upgrade list on their website?
Answer: ...after an exhaustive search through discussions at FlyerTalk and Milepoint, blog posts at The Cranky Flier and Boarding Area, and articles at other places which report on the vagaries of AAdvantage and its tools, I'm prepared to say that as of May 2015 the answer is still no...
There seems to be some other methods we could use:
Argument from authority
We demonstrate that reasonable, knowledgeable scholars believe that "no evidence exists". We can do this by citing references, or just wait until such a scholar decides to answer the question themselves.
A logically valid argument from authority grounds a claim in the beliefs of one or more authoritative source(s), whose opinions are likely to be true on the relevant issue. ... it is likely to be true, rather than necessarily true. (Rational Wiki)
The fact that these questions can be authoritatively answered by a scholar, may even attract scholars to the site.
Reductio ad absurdum
There's also reductio ad absurdum, which can be applied in the context of Islam, e.g.:
"Is such-and-such haram?" "If it were then XYZ would also be haram. But Prophet Muhammad did XYZ [see hadith]. So no."
"Is such-and-such mandatory?" "If it were, then we wouldn't be able to pray. So no."
