The CI in VTK is handled using gitlab CI. One can add a job with new parameters, new way to build VTK and such.
Of course, in your case you would need to add MinGW64 as a new dependency, but that should not be too hard since it is all generated with a Dockerfile, also present in the CI.
All of that can be done from your MR (except for uploading the new docker image to dockerhub)
You will probably need help from Kitware folks to get it over the line but that is pretty doable.