Good article Bog_Myrtle. It looks like a great way to experiment with recipes. As you say it doesn't really suit all grain brewing but there's nothing to stop grain brewers using this system with extract to experiment with hops, spices, yeast etc.
I think the hydrometer would fit in the demijohn when you have filled it with wort. If so you could sanitise it, drop it in and leave it there for the duration of the ferment.
I thought your wit recipe was interesting. I thought I read before that Windsor would suit wheat beers - how did you find it?