Interesting! My goals are a little different. I have a 42 key atreus, and really miss the top row and outside columns. So I’m planning to keep all the keys. However, since I want to cut the size down I had planned to just drop the fn keys. As you mention, they account for a big chunk of the size of the keyboardio. I don’t actually use them that much, as I switch back and forth between standard keyboards, atreus, and the keyboardio, and haven’t gotten used to them yet.
I use the butterfly key and the escape key as function keys on my keyboardio layout. I should make up some cardboard mockups to see if I can’t find a convenient place for the fn keys. A swivel arm sounds neat, but could be fussy or breakable.
I like the idea of rocking the board to invoke the fn key, but that could be tricky when you have it on your lap. I also wondered about embedding a capacitive sensor on the front edge of the board, so that resting your hand on the edge would signal a function key press. That might also end up being a little fussy to get the sensitivity just right.