Especially near you ossa. I brother-in-law used to live near Dundee and do Tentsmuir Forest. He did "get away with it" but he also knew it was most likely illegal...
Looks like there's a few "forest drives", Land Rover tracks and not much else, and they're mostly over to the West.
I done loads of MTB'ing around Scotland, when I did a round of the Munros, and owned a house in Aviemore for a few years... Some great tracks...
I';m amazed there's no clubs that have managed to secure access on some of the Estates. I've driven my Land Rover all over Ardverikie (where "Monarch of the Glen" is set) estate, that was huge fun