Gary,
Never come across a diagonal which could be re-collimated.
I think the nearest you'll find was the Meade Flip mirror (in both the 1.25" and 2" sizes). These had excellent 1/10 wave full thickness mirrors mounted on an adjustable shaft and a stop fitted to refine the actual "flip" of the mirror.
https://www.mta.ca/uploadedFiles/Com...structions.pdf