Problem may be that the "usual suspects" we rely on for input in the copyright forum are on vacation?
Copyright's not really my area, but for what they're worth here are some initial thoughts uninformed by full facts or research (and you're right not to put details in a public forum):
In the context of academic texts, quoting a phrase, or even a few phrases, with attribution is pretty well recognized as coming under the "fair use" exception, I believe. So one could argue that putting quotes on T-shirts, T-shirts being a recognized medium of for political and other "speech", should likewise come under the "fair use" exception to copyright as an aspect of free speech.
On the other hand...playing devil's advocate...just putting the quote itself on a T-shirt isn't quite the same as when you quote someone in a written text, since in selling a mass-produced T-shirt you are profiting from the author's creation and not just making a reference to what s/he said to make a point in the context of a larger discussion. That fact could alter the "free-speech" aspect somewhat. (Depending on the nature of the quote, I could see an argument going either way.)
And, all that being said, as a practical matter you have to recognize that there are a lot of people willing to sue you even if you're squarely within the law, let alone if your position is at all shaky.
So, in the end, the risk-benefit of going forward would probably best be addressed by an attorney with experience in representing people who sell items that carry sayings--T-shirts, bumper stickers, mugs, etc.--who may have had to deal with copyrighted quotes and have a better sense of how likely the copyright holder is to complain and how successful such complaints have been in the past.