
1): Back up your scenario.
2): Play the scenario, and fast forward to June (to make snowline retreat a fair distance)
3): Save it as something (rock.sav) and quit OpenTTD
4): Change the .sav file to a .scn file
5): Go back into OpenTTD and into scenario editor and find rock.scn
6): As long as you don't change the year, you should still have a high snowline (changing the date will reset the snowline to January)
7): Now, place rock everywhere where snowy trees appear (I would recommend not putting rock on the tiles that have small specs of snow on.

Sorry if this seems a really simple method, or maybe even if there is a simpler way of doing it, but I enjoyed it anyway. Also, I have a hidden agenda here. I am thinking of putting this into my South Germany, Switzerland and Italy scenario, so I wanted to gauge public opinion. The crucial thing here is that the rock will never be seen, as after June/July the snowline begins to descend again.