Forget the above.
I found out!
There's an entry in module_scenes:
("random_scene",sf_generate|sf_randomize|sf_auto_entry_points,"none", "none",
(0,0),(50,50),-0.5,"0x300028000003e8fa0000034e00004b34000059be",
[],[]),
In the hex string. 5 digits starting from 12 should be changed to '691a4' - this apears to be max you can set in in-game terrain editor.
like: 0x30002800000691a40000034e00004b34000059be