Mutation Spell now can affect your slave's racial makeup (but not their actual race itself). Just download one of the patches and follow the instructions or enjoy in the updated main files above.
I woke up with an idea and 6 hours of work later I came up with something I'm proud of (aside from how long it took me) and thoroughly tested it. Works on hybrids, but does nothing to purebreds. I could change that if people want it... but for now, there it is.