The frame hinge is very close to the mounting area and so probably many cages won't fit.

I would consider something like this:



This way, even if the end of the water bottle is close to the headtube, you can still slide it out sideways.

That doesn't look as though it'd fit to me. There's only 20mm between frame hinge protrusion and lower cage bolt.

How about a water bottle cage that mounts to your handlebar:



Looks like it would provide easy access

Nah, that'd interfere with the fold. I can't remember who it is here who has a heavily upgraded Vitesse in red with 2 bottle cages (additional one underneath the main tube). I saved the pic of the bike as I liked the component choice but I can't read the logo on the bottle cage.

here what i have for my vitesse... for only $6 + tax hehehe

https://www.rei.com/product/722708

How does that look folded?

Update: never mind- I just saw the picture on your blog.

hahaha good thing you visit my blog...

a regular bottle cage won't interfere with the fold.