Ask in the right way and you might be surprised. I ran a few trips with big sections of off-road in Dumfries and Galloway forest (100miles+ off road) , Isle of Jura up to the top to view the Correyveckan, and a couple from near Ullapool which were about 25 miles of (2) estate tracks. Those cost me a couple of bottles of port, a few grovelling emails, and some commitment in terms of our attitude and behaviour (Specific times, not commercial, in a group, sedate pace, no damage to track, etc etc) . I had another in the bag out to the end of the Scoraig peninsula but bad weather put a stop to that one (hurricane force winds on exposed cliff side tracks).
I stopped doing them after a dickhead declared UDI and shot off along the tracks at speed, wheel spinning and wheelying. You just can’t trust people and I decided I did not need dickheads in my life. Said dickhead is or was a member here actually.
There’s a website somewhere that tells you who owns what in Scotland. Find out the estate, look the estate up - they’ve nearly all got websites as they are commercial outfits these days then either ring or email the land agent, or the ‘factor’, or the game keeper/head stalker/ghillie. The actual owner is very unlikely to be involved. At least try to sound like you know what you are talking about, eg stalking & shooting seasons will almost certainly be a no. Aim to make it a low impact event and if they get anywhere near to agreeing just agree with everything they want. First time do it exactly how you said you would, after that you can always ask if you can do a bit more. On the Ullapool one’s the second time the agent of the second estate basically said go where you like on the estate tracks. Finally make sure the local police know what you are doing. It’s just a phone call.