If you have a pentaprism and a track or slide ( which does not need to be ultra-precise), the pentaprism or pentamirror can be moved across various diameters of the flat under test. An autocollimating telescope is aimed at the prism from outside the rim of the mirror. The image of the reticle in its focal plane is bent 90 deg. by the prism, reflects off the mirror, and returns to the focal plane via the prism. The variations of position of the returned image are a measure of the slope of the mirror surface at several points across that diameter. Thus , a map of the mirror surface can be constructed.

This seems laborious and repetitive, but I recall its being shown in the optical production book by D. F. Horne. or possibly elsewhere. Perhaps it can be semi-automated.