I think we can treat such cases quite analogously to requests for translations of single words and require previous research effort.
In this case a minimum research effort would be consulting a dictionary, which provides phonetic notations (usually in the International Phonetic Alphabet) of at least most words. Alternatively, as the German orthography is phonetic (though in a complicated way) for non-loanwords, it may act as a phonetic notation for them.
If the asker reports his findings on the phonetic notation or his deduction from the spelling of a non-loanword and says why they are not satisfying, the question is on topic.
Finally, analogies to other words might help, however, if they fail to yield a satisfying result, the problem is likely not limited to a single word but of a more general nature – which makes it a valid question.
Now, what if the asker does not have sufficient knowledge of German to use use any of the above? Then the actual question can be seen as equivalent to a more general one, for example:
How to pronounce definieren?
might turn into:
How to pronounce words ending on -ieren?
What are good ways to determine the pronunciation of an unknown German word?
If we decide on this, we should extend our FAQ with instructions for asking questions on pronunciation and in particular have a question like the latter to link it.