Dear Matthew,
It is an interesting question and I'd be curious to see what others say. The category of "rhetorical question," I suppose, includes questions to which the speaker does not actually expect an informative answer, including questions in which the answer is implicit in the question. I have seen praśnavyājēna, "in the guise of a question," but not in this sense (rather when one asks a question in order to distract someone from some other topic).
One interesting approach is offered by the anonymous commentator on Śaktibhadra's Āścaryacūḍāmaṇi. He distinguishes surface meaning from "final meaning" (vākyaparyavasānam). So Rāvaṇa's verse at 3.17 (yudhi sarabhasaṁ hatvā rāmaṁ balān mayi gr̥hṇati svayam anucitā bhartuḥ śōkād asūn na kim ujjhati) is a question, on the surface, which the commentator answers (ujjhaty ēva). But he then explains the presumptive answer (Sītā will die if Rāma is killed) as a reason for a further conclusion (therefore Rāma ought not to be killed). That conclusion is the final meaning (yata ēvaṁ gr̥hītā sā prāṇān parityajati atō rāmahananaṁ balād grahaṇaṁ ca na kartavyam iti vākyaparyavasānam). So a rhetorical question might be one which has the form of a question but has the "final meaning" of something else, in this case a prohibition. The reverse happens sometimes too, where something that has the form of a statement is read as a question in its "final meaning" (e.g., "I see that you're the only one in this ashram" > "why are you the only one here?").
Andrew