I like the idea here. Are you on 2.3.1 or are you on the latest development version? There have been improvements to the RMA algorithm that speed the process up, but if you have too many tiles with complicated connection information, sometimes the algorithm can't find the right setup. It's worth bearing this in mind with your canyon.
You asked in IRC I believe about the size of an RMA. Some RMAs are assembled into 10x12 so you should have plenty of space.
My recommendation for keeping the connections simple is to just use two definitions:
a: connects to low ground
b: connects to high ground
Remember that the definitions are only important for how the connect to each other, so a tile with all "a" could have a rock, a ufo, a cactus, whatever you'd like.
// Tile with a canyon that runs vertically with high walls on either side
tile +vertical
{
6 6
0 b a a b 0
b +b +a +a +b b
b +b +a +a +b b
b +b +a +a +b b
0 b a a b 0
}
// Tile with a canyon that has high walls on the left but the low ground wraps around on the north side
tile +corner_sw
{
6 6
0 a a a a 0
a +a +a +a +a a
b +b +b +a +a a
b +b +b +a +a a
b +b +b +a +a a
0 b b a a 0
}
Another thing to keep in mind is that from the player's perspective, it often doesn't matter if you have vertical AND horizontal variations. A canyon running vertical and a canyon running horizontal is identical in the game because the player can simply rotate the view. You only need vertical AND horizontal variations if you want to include both in the same map (ie - a corner, a t-junction, etc.).
