All connections must be in accessible boxes.

Did you say you were feeding the transformer
thru a dimmer? I don't like that idea.

"All connections must be in accessible boxes."

Not necessarily on the low voltage side.

The bigger problem is usually that all concealed wiring must be an approved type (even on the low voltage side) and that a single transformer often leads to VERY large wires.

Using a number of separate 30 W power supplies is often easier.
They are small enough to mount under the cabinets, and then the wiring is 'exposed' (it can be inside plastic covers) on the bottom of the cabinets.
The supplies are also capable of being dimmed using a regular dimmer on the 120 V input side.

The only connection under than cabinets that requires a JB is the 120 V to the power supply.