That goes to heart of the manner. With little doubt, Civ5 is much simplified and streamlined. The question is can the game be modded to be less so. For instance, can the really simplified global happiness mechanic be modified back to individual city happiness? Of this I doubt because this is a core mechanic now.
To answer this question in a purely technical way, it probably depends on what's under the hood in the DLL file (which we have yet to be able to edit, from what I've heard.) ModBuddy does not work for me or I'd consider looking in there to see if its doable in Lua. As it happens I did some heavy DLL modding in Civ IV, and at one point added some extra +Happiness features to the core engine. Here's what we're up against if Civ V works like Civ IV:
Global happiness actually did exist in Civ IV, but it worked differently. On a pure data level, every City and every Player object had several dozen happiness variables stored on them. There was a variable for each individual thing like Resources, War Weariness, Traits, etc. Then each variable was further subdivided into good or bad, so you'd actually see GoodResourceHappiness, BadResourceHappiness, GoodWarWearinessHappiness, BadWarWearinessHappiness, etc. The reason for tracking it to such a granular level was that the game interface reports a breakdown of each individual source of Happiness and Unhappiness (i.e. "bad happiness") when you mouse over the display in game. Civ V does this just like Civ IV, so I'm hopeful it's set up the same way.
If that's the case, to make it work like Civ IV, you'd need to find the portion of the city code that does population growth. We'd total all of the Happiness being generated in that city from the various sources, then combine it with the Happiness being generated empire-wide (likely stored on a Player object). You then check to see if this number is less than the city's current population. If it is, allow the population to grow that turn, if not, don't.
The next part is reporting to the player what's going on in the city. To do this in the city screen is just a matter of changing the Lua code for the screen so you include a section with the happiness number. You'd probably also want to write a script that displays how that total is calculated. You'd also probably want to find the screen that shows statistics per city and at least modify it to show the total happiness in each city.
Right now I'm not totally sure which parts of this are exposed to Lua. If it works like Civ IV, the stuff that's needed to change the city screens is already in there and does not require a DLL recompile. The only part I'm not sure about is limiting city growth when the city is Unhappy, because I don't know if City::doPopulationGrowth or whatever it was called is an exposed method. If it is, then if I can get ModBuddy working I may volunteer to build a prototype for folks. That's a big if though, since ModBuddy is telling me I'm "missing resources" even after installing Visual Studio components.