So there are at least two examples, if one is overly confusing, cross reference with the other :)

Oh, TreeDropdownField only expects a Hierarchy I believe... but that's difficult because it's not an interface (it's an Extension; capital E). As to what it actually expects (read: how coupled that is to SiteTree) I dunno. But it works with more than just SiteTree (or at least used to) - in that it goes for Folder as well.

I've just been trawling through the code which renders the tree... I think that's one for another day!

Yeah, I sorted the Hierarchy bit out - that was just me being dull

But in terms of how much is coupled? I dunno. I recommend sleep first.

Heirarchy is applicable to anything, and SiteTree uses it to do just that.

So, in answer to my own question. The SiteTree 'tree' is a big, scary thing which I'm way too tired to even consider playing with tonight!

The mistake earlier was a little harder to spot... that was a closing bracket in the wrong place. Not wrong enough to throw any kind of error or warning, but wrong enough to not actually work 😛

I missed a parameter when I added the TreeDropDownField