They work and certainly hold the 4th axis securely, but in the long run I'll probably make up a custom clamp as these are a bit chunky as a semi-permanent fitting.
To celebrate I mounted a bit of 1" bar and mandraulically milled a square and a triangle, really just using the 4th axis as an indexer. Although the brake is fitted and worked well, I've not yet run compressed air to a convenient point, so this was done with no braking. No sign of chatter what so ever so for many things I probably don't need the brake - but I will run the air anyway.