They're hard to find, but I've finally tracked down a place in Australia (mechkb.com) that will internationally ship me one of these convertible Cherry MX key babies, that supports both Mac and Windows, both bluetooth (pairing with four devices) and USB (for low latency gaming and charging):
http://www.keyboardco.com/ sells the Filco Convertible 2 [0] and a few other Filco keyboards - they also ship internationally. Bought from them before, would definitely do it again.
That's a great store, but they were out of stock, and nobody selling them on Amazon would ship to the Netherlands. Maybe they're easier to find in Japan, where they're made. I made sure to lock in my order from Australia where they had one in stock, before blabbing about it on Hacker News. ;)
It seems so sensible, I don't understand why there aren't more convertible keyboards like that! Together with a convertible bluetooth mouse like the Razer Orochi that also supports multiple bluetooth pairings, it's the best of both worlds for low latency USB gaming, charging, and wireless switching like a KVM between Macs, PCs, tablets, phones, GearVR, etc.
Be careful shopping for convertible devices that actually use Bluetooth and not some proprietary dongle, and that actually use USB for fast low latency data instead of just charging!
Filco Majestouch Convertible 2 review: the quest for a Mac-compatible, wireless, mechanical keyboard: http://johnscullen.com/filco-review/