Of course it is. Available choices at scenario start can be limited to one to guide player to choose the right one. Saving and reloading will give all valid choices as selectable, but then again player already knows what he / she should pick. You can have event condition that it fires only for specific TAG and as counties are tags too, you can have events that fire only for one county. Of course as events are random you can't ever be sure when it triggers, if at all. Biggest job is likely scripting all those events.
Of course if player would change primary title in any way (by creating a higher level title for example), the events wouldn't fire for him anymore. I think it would be best if player would need to choose one specific duchy and be given specific instruction to not to make him(her)self king or perhaps choose a kingdom (could be small one), which would remove the possibility of primary title changing (except if lost in a war).
