The system hangs on boot when replacing /usr/sbin/installer and OSInstaller in the 10.15 BaseSystem.dmg with those from 10.13.6. The install is successful and I can then ditto the files into the right places, but the device hangs on boot after the install is finished.
I'm trying to replicate my accidental success by installing from my active 10.13.3 install. If this works, I'll have some more info to go on.
Edit: It didn't work. I'm going back through my history to see what I did differently.
Conclusion: it’s still not working. Either I did something vastly different that time, or I didn’t erase the disk after the previous attempt. I can’t say and I can’t recreate my error. I didn’t save the terminal output because I wasn’t focusing on it. Ah well, back to the drawing board
Even if you create a working Catalina HFS+ installer or adding a manual script to pkgutil the Core.pkg to a target volume, then you still have to move Template/Device/ and Template/Users/ to the HFS+ / root, so the Installer won't automatize that process, as it was for Mojave.