Here is another possibility from the O Gauge discussion forum,
https://ogrforum.ogaugerr.com/topic.../video-z-stuff-dz-1012-block-detector-controls-two-o-gauge-trains-on-1-mainline
Why I was banned from their forum is beyond me, but attached is the image using a Z-Stuff relay 1008A and block detector, very simple and upfront. If I am correct the relay is a SPDT, if they are hard to come by this might work, would have to figure out how it compares to the Z-Stuff relay.
Should be a very common item available at a reliable jobber auto parts store such as a locally owned NAPA.
85 and 86 are the coil, (red and black) 87a is NC, (white) 30 is common (blue), not sure what the green wire is for on the Z-Stuff relay.
85 to number one on the bussbar
86 to number two on the bussbar
30 to number four on the bussbar
87a to number six on the bussbar
The Z-Stuff signals all appear to be wired the same, one would not have to use the block detector, they could use one of their other signal towers.
Feel free to critique, throw out ideas, corrections,