The specific point where they get angry is when you do a sweeping reveal that, gasp, "it was [hated bill] after all!", though, right? The question doesn't have to come with a "punchline"; you can just poll people once for their opinions about named bills, and then again separately for their opinions about un-named bill contents, and correlate these, without ever revealing to the second group what bills the quoted language comes from.

Or do you mean to suggest that people get angry immediately when they realize they're being asked for their genuine opinion about something which might turn out to be something they're expected to toe a party line about? If so, that's a very interesting effect, possibly a chilling effect to any potential for genuine informal conversation about these bills.

In my limited experience, people get angry when they feel tricked, so the former of those two suggestions. I agree that you could not tell them and get some good info.