250.30(2) does not speak directly to the question of a CD disconnect in the transformer enclosure.
It says "If the source of a separately derived system and the first disconnecting means are located in separate enclosures, a SSBJ shall be installed with the circuit conductors from the source enclosure to the first disconnecting means enclosure.
In my case the first source disconnecting means is in the source enclosure. So an interpretation is that 250.30(2) says my SSBJ requirement has been met in the first enclosure.
Is this an interpretation others would agree with. If so, I am going to have to make a code reference note in my specification as I am sure I will be questioned on this by some inspectors.