They make some that work upside down. I've got one, if I can find it...
- edit -
found it
View attachment 3996913
Unfortunately, the design still isn't perfect for spraying the undersides of leaves.
There's some type of valve at the top of the stem which allows fluid to pass if the bottle is inverted, and does not allow air when the bottle is upright. Unfortunately the best position for spraying the undersides of leaves is with the bottle parallel to the floor, and the transition between top & bottom causes bubbles in the line.
Still works, but not as well as I had hoped.