newt,
I think there are several points here.
First. I'm not aware of a readily available listed dimmer, that can operate on a 12 (twelve) V AC source, that can be field installed as part of the Premises Wiring (System) as defined in Article 100.
I'm not saying it doesn't exist, rather, it is probably uncommon and expensive and difficult to install if it does exist.
Second. Is the transformer that powers your low voltage lights a light weight electronic supply, or a heavy magnetic transformer? A magnetic transformer is generally dimmable with a dimmer rated to work with magnetic transformers. As Jeff noted above, they are not cheap.
Third. If the low voltage light power supply is electronic, only the manufacturer of that supply can tell if the supply can be dimmed, and whether a certain type of dimmer is required. Some electronic power supplies are dimmable by standard dimmers, others by "electronic low voltage" dimmers and yet others are not dimmable at all.