OrnitheMC is a toolchain to bring Fabric/Quilt loaders to old minecraft versions instead of forge, this allows not only a fresh take on modding, but also to use newer Java versions with old versions!

It would be nice to be able to use CPM on it.

I am perfectly willing to do the ports myself and fire a PR your way, if wanted

Can you use the existing MCP mappings with it, if so code can be shared between the Forge version and this.
If not, no worries. Just keep the project setup the same. I have a loom plugin to make MCP mappings work MCPHybridLoom I use it for 1.14-1.16 fabric versions, you probably have to adjust it if you want to go with that route.

Use relative linked source folders, instead of multi project gradle example.

If shared code can be done:
See the modern versions on how to make version specific shared folders create example, use example Create the shared folder in the Forge project.