Nice work msmith!
What's your idea on saving the terrain?
I believe if you do random_seed(my.x+my.x) whenever you create a new entity at (x,y), then you will get unique numbers for each tile, that are the same every time you run past that tile.
You might have to take the absolute value of the thing though, in which case you might get a point-symmetry around the origin. To avoid this, you can try:
random_seed(absv(my.x+my.y + sign(my.y)*my.x+ sign(my.x)*my.y))
or something like that.
random calls after that will always produce the same numbers, but only for the same tiles.

This means you don't have to save anything. Or, if you do want to save something, at least you don't have to save everything