Hi guys, I am no noob to mapping, but I have never modded, so this is uncharted territory, please be gentle. ;-]
Is it possible to mod a terrain tile (or subtile) to block the vision of units (or vision and movment) while remaining completely transparent (the terrain tile that is)?

Yes t is possible how I am not quite sure being a noob modder myself.I know it would have to deal with tile edit and something else.I would ask farty or corbo they know more then me.

It is possible to make terrain megatiles, tiles and minitiles to block vision and movement.
However I have no idea what you mean with "Transparent" terrain.

All you'd have to do is copy the terrain tile into a null unused tile (so you'll get two tiles with the same appearance) and modify the flags of one of them to block vision. Then on the editor place one tile or another if you want vision blocked or not.

Oh, I get what you mean. You want to be able to put the transparent vision blocker ontop of any tile you want, so you can block vision wherever you want.

Exactly
I guess that would work, but since I'm trying to do it to the top of every cliff on the map, that would be too painstaking!

I don't think there's any other way around...

There is not.

Doesnt the cliffs auto block vision