This was a bit tricky, as byzantium is still an in-development release and all changes should go through landing. Fortunately though, the synchrotron Laniakea module is flexible enough nowadays to accommodate for that.
I've implemented a solution which will fill up the -updates and -security suites directly for byzantium from the respective bullseye suites, while not allowing manual uploads from us which should still go through landing. That workflow should work well for byzantiums current odd in-between state between "released" and "in-development".