So I've been using my keyboard for quite a while now, and - aside from the occasional double input - the TextBlade style 'spacefn' layer switch works fine. In fact I use the right thumb key as space/layer switch, and the left thumb button as back space/edit layer switch (home row arrow keys, cut, copy, paste etc.).
I'm pretty happy with how this first prototype worked out, and am considering a new version with an expanded thumb number of thumb buttons.
The name Katana comes from an interest in Japanese Samurai writings (Ronin anyone?), but it just happens to nod to the text BLADE