Just got the following email from Andrew Wang @ Tommo in response to my aspect ratio question:
Originally Posted by Andrew Wang:
Thank you very much for your interest in our product.
There is indeed an option to play the games on the NEOGEO X Handheld in 4:3 aspect ration[sic], in this mode some portions of the screen are unused.
WOO HOO! Fears (mostly) put to rest!