@Jimehc: Thanks for replying.
3 more builds and a bunch of poking around and it gets even weirder. My SIB config regfile that I integrated into the build is there in the PE registry when I boot up. But SIB is ignoring it somehow. I have to open StartIsBackCfg.exe and go through it, noticing that 90% of my config choices are already selected - but NOT reflected in SIB - and then when I hit OK SIB is suddenly configured as I want.
Can anyone tell me what's going on?