San Diego has Black Crake, and they hold Guam Rail off-exhibit. I saw lots of Common Gallinule at the SDZ Safari Park, but I believe they were all wild so I don't think they count. Interestingly enough I don't remember hardly any small Gruiformes from those two zoos, despite their large bird collections.
Oregon Zoo may have had Black Crake I think, not sure if they still do.
Woodland Park has Gray-winged Trumpeter.
I remember Fresno Chaffee Zoo having Gray-necked Wood-Rails, as well as Guam Rail on exhibit 5-6 years ago.
@ChaffeeZooFan might be able to confirm whether these species are still held?