Lightmapping and Diablo3-style big-tile levels

We’re working on a game that will have procedurally generated levels. One of the approaches to level construction we’re considering is using big tiles like those used in Diablo 3

Tiles in Diablo 3 seem to use some form of lightmapping (at least the ambient occlusion is clearly visible). The game reportedly runs well on low-end PCs (with dynamic shadows turned on), so it’s unlikely that it does lighting in realtime.

The question is, how can we use Unity / Beast lightmapping for the big tiles, and how do we combine them into levels at runtime?

create a prefab for each tile
then do instantiate prefab
create a grid (= array of arrays)
and everytime you run, you select each tile/prefab
and instantiate it on a random position in the grid
do a few checks to make sure you don’t place 2 tiles on each other etc and your done