The American standard is to use double quotation marks ("example") and the British standard is to use single quotation marks ('example'). Style guides insist that you should be consistent regarding which mark you use, regardless of circumstance.
My question is whether or not it is acceptable to use the alternate quotation mark to denote something that is a quote, but is not dialogue.
For example, as a kind of joke, I wrote:
'Of course I know,' he probably would have said.
The "he" is an animal, incapable of speech. I know that readers sometimes jump from quotation to quotation, ignoring attributions, so I used single quotation marks to make it more obvious that he wasn't actually saying those words. Is this acceptable?