I'm using the super polished version of Dreamcast by Performace, and I've used Altivec whenever possible. I'm not a programmer, but I believe that if the community comes together, we can release a stable version that runs decently on a G4.
There is more information about the project in the readme.
https://github.com/Rikintosh/re3-powerpc
There is more information about the project in the readme.
https://github.com/Rikintosh/re3-powerpc