Mixins are way less effective/compatible than ClassFileTransformer for Redirector.

Currently investigate how to register the transformer

https://github.com/MCTeamPotato/Redirector/blob/modern-fabric/src/main/java/com/teampotato/redirector/RedirectorTransformer.java

https://github.com/FabricMC/fabric-loader/blob/master/minecraft/src/main/java/net/fabricmc/loader/impl/game/minecraft/launchwrapper/FabricClassTransformer.java
I was referring to this class earlier.

So if we create a transformer impl ITweaker like FabricTweaker, how can we register it and get it loaded into mc somewhere?

Fabric use launchwrapper, so use ITweak

use Launch.classLoader.registerTransformer in some early stage. ITweak is a early stage, but not the only one.
You could ask fabric for help. Find out how to create a coremod.
I am concerned that coremods are not supported, since fabric is built in mixin.(x