See, this is something I'd be keen on.
I think 3e's shift to granting majority XP to killing monsters is what lead to all classes being weighted by how well they killed things, and removed the premise that different classes were good at different things. I suspect that giving an XP option beside just killing will emphasize a rogue's trap-beating abilities, fer instance.
The problem I have with arbitrary story-based or milestone XPs is that I prefer a more granular reward system where PCs get experienced for specific things rather than reaching chapter 2 in the story. Personal preference completely at play here, but I'm thinking an XP fer Gold module would meet my needs.
Here's a thought. I don't have the monster manual here, so I'm going to make up some numbers:
Let's say there's an encounter with 6 Rocklobbers, and each is worth 200xp. The party would net 1200xp for killing them all, while carting home their treasure of 20,000gp.
If we move all that experience to the coins, the Rocklobbers are each worth 0xp and the 20,000gp are worth 1200xp (or roughly 1xp per 17 gold coins).
Now, if we gave some credit for killing the Rocklobbers, I'd say 1/10 listed XP would be fine, so 20xp per lobber takes up 120 of the assumed 1200xp of the encounter, leaving 1080xp to divide among the 20,000gp (or roughly 1xp per 19 coins).
In a perfect world, I'd run that same math across a number of different monsters and see if there was a mean score that I could comfortably apply across the board.
Ultimately that should even out with the assumed wealth per level (the PCs are getting the same XP, but for bringing home treasure rather than killing monsters), but with the reward being on getting the prize rather than slaying the guardian.
To be honest, I'll probably wait for somebody else to do it and post it on the internet.