Well, you could add the trigger NOT = { desperation = { country = BEL data = 30 }} to the londen treaty, and make it persisten until slept
Then it will only fire if Belgiums owns 70% or more of their territory, and if it fires it should sleep the Dutch victory event, and if the dutch victory event fires the London treaty should be slept... I think that it will be quite hard for the Dutch AI to control 30 percent of the Belgium territory before the London treaty will fire, however, if it would be too easy when you are trying you could turn it to 40 or even 50 (though I think occupying 50% of Belgium befoere the London treaty fires would be impossible for a human player too), also as I said before the province demands in the annexation event should be replaced by a desperation = {country = BEL data = 100}
