Hi! Schoolhouse are definitely the place to look, as they have all the legal stuff which is specific to Scotland, and as you're already aware that's different from the situation in England www.schoolhouse.org.uk/
It's also worth reading through the official guidance - many councils either don't know the legal side, or don't care, but if you have a copy of it handy and quote them relevant sections, you are more likely to be able to prove your point!
I didn't have to go through asking for permission because DS wasn't in a state school when I took him out to HE, and even though our council knew we were doing it (we had to phone several times to work out that we didn't need permission, so they had our details) they never asked us for an education plan, though they could have if they'd wanted to...
So, I can't speak from personal experience I'm afraid.
But I dug into it at the time in case I suddenly had to write something up, and was going to base it on the following stuff from the legislation -
Consistent involvement of parents or other significant carers.
Presence of a philosophy or ethos (not necessarily a recognised philosophy), with parents showing commitment, enthusiasm, and recognition of the child's needs, attitudes and aspirations.
The opportunity for the child to be stimulated by their learning experiences.
Involvement in a broad spectrum of activities appropriate to the child's stage of development.
Access to appropriate resources and materials.
The opportunity for an appropriate level of physical activity.
The opportunity to interact with other children and adults.
If you can write up something which shows how you plan to deal with each of those things, I don't see that they can argue about it not being adequate. I'd suggest using the list as paragraph headings, then following each with a description of how you are going to achieve it. Under "broad spectrum of activities", I'd then subdivide into whatever categories suit you - if you were going to be autonomous, then you'd probably list the kinds of things you were likely to do, and how they would be educational. But if you're going more structured, you could divide it by the areas of the Curriculum for Excellence (Expressive arts, Health and wellbeing, Languages, Mathematics, Religious and moral education, Sciences, Social studies, Technologies)
There are also the principles of the Curriculum For Excellence (bit like English NC) -
Has opportunity to develop as a successful learner, effective contributor, confident individual and responsible citizen.
Knows they are valued and supported.
Has opportunity to develop skills for learning, vocational skills and skills for life.
Has opportunity to develop a range of knowledge and skills that adds up to a general education.
Finds learning relevant and meaningful.
Is challenged and engaged by their learning.
I'm not sure how you would write anything up on these "principles", as they're a bit vague, but I'd be tempted to write something to show that I at least knew about them.
All of this is probably overkill, and I have no doubt they ought to accept far less detail than what I'm suggesting, but I tend to assume they are going to have a list where they have to tick all the boxes, and that list is likely to be based on what the guidelines say, and on the Curriculum for Excellence, so I reckon if you structure what you say like this then you will make it much easier for them to connect your statements with their tickboxes and give them less to whinge about!
Oh one last thing - before you start writing up a document, do send them the letter and get their "pack" as the pack may include a form for you to fill out with these kinds of sections already on it, or some other set of sections - legally you can always do it as your own letter instead of a form, and they can't hold it against you, but it can be easier to "pick your battles" and do it their way as long as they aren't being excessive about what they ask of you, rather than starting out being seen as "difficult" until you have to dig your heels in on something. Hopefully if you make it easy for them to tick their list of boxes, they'll just grant you permission without too much difficulty.