I'd be curious what percentage accept/reject on the spot, but I'd never ask a candidate to provide an answer either way if you present an offer right away. It's too important a decision for them not to at least take the night (especially after a day of interviews, a flight/hotel stay, an impending flight home). Even suggesting an immediate answer feels salesy ('whaddya think?'), and I'd think we'd see better results by presenting the offer and saying "take this home with you and get back to us when you're ready".

This approach should also tend to reduce counteroffer acceptances or accept/renege scenarios.

I like asking the question, "if we were to extend you an offer you like, would you accept it?" It's not perfect, but I haven't of a better approach that isn't pushy.

How about "Is there anything in your situation right now that would prevent you from accepting a solid competitive offer?" That's a little less salesy IMO.

If "not illegally confining someone" is all it takes not to be a "high-pressure sale tactic", then I confess I don't understand the term.