But if a deity produced something which caused everyone who saw it to say "Hey, that's a square circle!", then they would have produced a square circle, wouldn't they? And that's certainly logically possible. With omnipotence there's no clear boundary here between doing something and making everyone think you've done something. If their thoughts are convincing enough, then you've done it.
		
		
	 
Seems like more of a philosophical question than a scientific one, not that science was ever mentioned as a qualification.  I tend to disagree with your assessment on the principle that everyone thought the earth was flat until sufficient evidence came to light to demonstrate that the earth was, in fact, spherical.  The illusion of a flat earth was completely compelling, but it was false.
I believe that the spirit of the question "Can god create a square circle?" implies that the end result of the satisfied condition is an actual square circle, not merely the illusion that one exists.  But that's just me, I don't claim to speak for everyone.