You're right, MPPs ARE something to look out for, but that's now how they work. MPPs are only triggered when:
-The workers of a member nation are captured
-The units of a member nation are attacked or bombarded
-The territory of a member nation is bombarded
These often happen right after a declaration of war, but a declaration of war itself doesn't trigger the MPP. If you really want to play with AI reputations, you can find a nation you want to attack that has lots of MPPs with other nations, make MPPs with those nations yourself, then declare war on the nation you want to attack. Then send a worker or two or a sacrificial warrior you've still got around from the ancient era to a spot where they could easily be captured or killed by the nation you declared war on. Don't do any other offensive action. Despite you being the aggressor in the war (by declaring war in the first place), when the AI you're at war with captures that worker, all of his (former) honchos will turn around and declare war on him, too, since your units got attacked first. It will also put a huge black mark on the reps of every nation involved (except for you, ironically), since in order to honor your MPP, they had to break the one they already had. Conversely, if you made all the MPPs, then went and bombarded his oil source first, then you'd be on the receiving end of all of your former MPP allies, however your rep would still be okay, since the allies are the ones that are considered to have broken their treaties when they sided with your enemy. They'd just all hate you because you fought a war with them.
But yeah, in general, beware AIs who demand things while in a bunch of treaties. They're nothing but trouble. And if you have the option of making a counter-proposal, that's not a threat, that's the AI initiating diplomacy with you (though the quality of the deals you'll get from an AI might make you *feel* like you got extorted).