The tap rules are basically saying a tap conductor can be protected at the load end at it's rated ampacity. You are describing a feeder tap. The issues are just that you need to follow the appropriate tap rules for the length and wiring method.
If this is 25' or less we use 240.21(B)
(2) Taps Not over 7.5 m (25 ft) Long. Where the length of the tap conductors does not exceed 7.5 m (25 ft) and the tap conductors comply with all the following:
(1) The ampacity of the tap conductors is not less than one-third of the rating of the overcurrent device protecting the feeder conductors.
OK it is half of the main
(2) The tap conductors terminate in a single circuit breaker or a single set of fuses that limit the load to the ampacity of the tap conductors. This device shall be permitted to supply any number of additional overcurrent devices on its load side.
(3) The tap conductors are protected from physical damage by being enclosed in an approved raceway or by other approved means.
This is the open question, is it?
I agree the "tap" is actually the only load on the "feeder" but I don't see anything that prohibits that and it would be possible to add another load later.
I am open to another opinion tho.