I've been in touch with a couple of people at Scottish Natural Heritage, who look after 1,400 SSSIs (Sites of Special Scientific Interest) in Scotland, in connection with this new cache. Permission was readily granted by the local officer with responsibility for the Muirkirk Uplands. In his words:

SNH doesn't have any policy on this particular activity, possibly because it is pretty new (as far as I am aware, you may be the first to mention it to SNH), but I would reckon that it is covered in general terms by the Scottish Outdoor Access Code (see http://www.outdooraccess-scotland.com/ ) which explains how our legal right of access to the Scottish countryside is entirely dependent on responsible behaviour.

I can confirm therefore that SNH has no concerns at all over the activity and I would very much doubt that you need to contact any land owners.
Which looks pretty positive for any future SNH inquiries. :socool: