One complication is that there's no longer Policy Opener Policies for the three ideologies like there is for the other policy branches. So you'd have to make the lua look for whether any one of the '1st tier' tenets had been adopted, and then look through cities for Building X, and then grant the dummy building with the +15% great people generation in the cities that had Building X.
In strictly XML code there's no way to do it since there's no <Policy_BuildingSomething> table that allows the increase to great people generation via a building.