] We see in many of JA's works that parents held a near veto right over their children's marriages - it was rare and imprudent to marry against your parents' wishes.

Not entirely true- if the parties were of legal age, they could marry without parental consent. That's why Darcy can ask Lizzy to marry him without asking Mr Bennet's permission first.The parents couldn't stop the marriage- they could only withold any fortune, etc. which the children might expect to inherit.

If a man were to propose (without having cleared it with his parents first) and be accepted, and then his parents refused permission, could he use this as a reason to break the engagement without risk of legal action?

He would be morally bound to reveal the fact that the expectations he had when proposing marriage could no longer be relied upon, and therefore the lady had to re-evaluate her acceptance.

Isabella evidently believes herself engaged to Captain Tilney (otherwise she would not have broken her engagement to James Moreland) yet he (Cpt Tilney) 'escapes.'

I disagree... I think she breaks her engagement to James in the hope that Capt. Tilney will propose.

I was wondering whether he could have used refusal of parental permission to defang potential legal action? (A problem with this is that he didn't ask for permission prior to breaking the supposed engagement.)

Well,I think that's hypothetical, since presumably he's of legal age to marry, and so is Isabella. And Henry makes it quite clear to Catherine that his brother is well aware that IT is engaged, so he can't claim ignorance.