I ended up writing a completely new system based upon grudges, which handles the long term effects of unpopular events. Im not sure whether I will 'refresh' grudges if they persist or just generate cumulative ones.

So for example if i whack tobacco tax real high, smoekrs will get a grudge against the government. Regardless of what else happens, that grudge will still be around, and will gradually fade out over time.
hopefully the effect of this is that its ok to tweak values, and make people a bit unhappy, but theres a 'tipping point' where you will give people a long lasting reason to hate you if you push past that level.

Now the usual system of affecting opinion in the game already has a variable 'delay' so if we raise school spending, people only get happy about it as it takes effect, maybe over a year or so. The difference here is that this is an additional layer of fading opinion, which is unaffected by future events.