Is there an allowance for a splice (inside a junction box) for a "Feeder Cable". The NEC doesn't seem to mention it one way or the other.

You are right, NEC doesn't say restriction for this, at least, I couldn't find it.
According to 110.14 (B), 215 and 300.13. But be carefull, some utility companies don't permit to do that before meters to avoid stealing energy.

It would be very unusual for a power company to be concerned with feeders.

They concern themselves with service conductors.

The NEC does not prohibit splices in either.

Power utilitys also arent required to follow the NEC. They are governed by I believe the NESC(national electric suppliers code?)