The Duo has it all in ROM, remember Apple announced a PCMCIA Dock and then didn't deliver, even after Newer had done the Design and had the Tooling done for the PCMCIA UltraDock. They wound up just using it for everything BUT housing the T-Rex/Card Cage.
I wouldn't be at all surprised if the 1400, 5300 and 2300 all use what is basically the same ROM, because they're very close being different packaging of the the same machine, IIRC. The Duo 2300c was released with the PB 1400, if memory serves.
We'll probably have to cook up a Dock DeclROM to do it though, but that's fairly well documented . . .
. . . for an Apple product/spec. :
