On a small selection of flights thus far, I've had no trouble putting the Checkpoint Flyer under the seat in front of me on planes. (Only medium-to-large jets so far, no commuters the size of an imported soda can.) My procedure, generally: just before flight, swap socks -- put previous socks in plastic baggie. On reaching my seat (hoping to be the first in my row), stash the bag under the seat, and then slip off my sandals next to it. (I would not do this with long-worn, stinky socks.)
What about the Western Flyer: has anyone used it as their under-seat / "small bag" second carry-on item (for itineraries and airports that make a 2d bag possible), or is that too crazy?