Okay after further testing, the custom maps still work, now with custom textures as well! Only thing missing for now is the skybox and having lava/water/trigger collider meshes. These will come in the future!
While it it fully functional and the build is up on Steam (it's been uploaded as a beta), I've not explained quite yet how to make custom maps nor how to upload them to the workshop. I'm doing that tomorrow. I had planned it for today but there were some minor issues I had to fix first.
If you really, really want to try to make custom maps today, you can hop on the discord server (link below) and I'll manually explain how to do it. Otherwise, tomorrow you'll have a fresh new guide + tutorial to show you how to do it easily.
https://discord.gg/nmRbPd7Y