Bar ends are a natural match for under-seat steering. They go on the end of the bar, and you just reach up and tweak them when needed. There appears to be fierce big-endian/little-endian debate as to whether the lever should point forwards (HPVelotechnik and others) or backwards (ICE), with forwards making for more natural ergonomics of the grip with a cable routed under it, and backwards providing a little more protection from rain entering the cable. Obviously the right answer is whatever you find comfortable.
They also work well on open-cockpit bars, in a manner akin to the drop bars they're designed for.
With hamster bars, my first thought would be thumbie converters, but that sounds a bit too thumb-y. End of the bars ought to work, though (much like the brake levers) you'll probably need them to rotate in the plane parallel to your thigh to avoid awkward knobbling. This will also make them relatively vulnerable to being bashed, but not as severely as they are on an USS bicycle (which tends to land with the weight of the bike on the pedal and bar-end shifter when it falls over). Alternatively, maybe you could mount them on a short bar end extension pointing upwards, or something?
Alternatively, this sounds like a legitimate application for electronic shifting.
FWIW, I've found the Microshift 9-speed bar end shifter to be more robust than the Dura-Ace one (which is vulnerable to cracking its friction ring when the lever is subject to excessive force from the bike falling over or bouncing against the bulkhead on a moving train, etc).