Short version: No, you can't.
Long version: Nooooooooo, yooooooou caaaaan't.
Very long version: In the XML, you can tie XP for new units to a policy (which is obviously global) or to a specific building in a city. And a building, in turn, could depend on the presence of a nearby resource. But there isn't an easy way in XML to tie it to a nearby Improvement or Feature.
In Lua, you CAN trap the OnUnitCreated event, figure out which city the unit's being created in, and look to see if this Improvement is nearby. This isn't that difficult to do, since this is one of the few Lua functions we know the syntax of, although the map check to see if a Citadel is near the city takes a little work. There's really only one little problem: XP awarded this way can't be used to pick a promotion until the NEXT turn. (Still working on finding a way around that one.) So it's possible in Lua.
Technically, you could also tie it to whether or not the tile is being worked. But the AI would have no idea that this benefit exists, and so wouldn't choose to work the tile. You could of course override this yourself in your own cities, but the AI's cities wouldn't know to do this.
What are the limits for modding in Civ5 compared to Civ4 right now?
The XML modding's not too difficult, and you can do quite a bit with it in terms of general balance tweaks. It's also pretty straightforward (although not FAST) to add new techs, civilizations, resources, units, buildings, eras, and such, with two main exceptions: 3D unit/building models, and adding new sounds (like the voiceovers you hear when researching a new tech or entering a new era). There are also a few things that, while in XML tables, are NOT easy to add new ones of, mainly Yields and font icons. But generally speaking, you can create a large amount of content using only XML, and it won't just be "more of the same" since there are a lot of unused-but-working stubs in the XML schema.
Lua modding of existing functions isn't too tough, either, although to do this you need to duplicate the original before making your changes, which make these sorts of changes the biggest source of conflict between mods. For instance, AssignStartingPlots.lua manages the placement of resources on a new map, so any mod that changes the resource rarities, adds a new resource, etc. (like several of the custom Map types) will be incompatible with each other.
If you want to modify existing UI elements, this sort of thing isn't that difficult to do, although you need to be at least a remotely competent programmer.
Lua modding of NEW functions is basically just yet another programming language; while you've got a huge array of possible things that can be checked and/or changed, the number of triggers is unfortunately limited at the moment. There's not an easy way to key an event to happen when a building is created, for instance, and several of the events we DO have triggers for are UI-related, where the event only fires when the active player (that's you) SEES it happen.
You can do a lot with this sort of Lua. Right now, the most common change seems to be to add new UI windows or pulldowns, with better information for the player. But you can also do plenty of gameplay-related Lua, like add new Wonder effects or some custom unit abilities. (For instance, in my mod, there's a unit combat class that has several units that adjust base power up or down a bit depending on the strength of the opponent. This is pure Lua modification.)
We're still waiting on the full DLL access, which'll open up the possibility of major conversion mods (like Fall From Heaven). Just remember that Civ4 didn't have that at the start, either.
while searching these forums I didn't find an 'API' for all the Lua functions either. 
