Even if one would see a scratch on the mirror, what matters is if one would see it at the focus plane, that is the groundglass. Keep in mind that the mirror in a SLR is acting at the unfocused part of the imaging beam, so next to any distorted rays will be enough plain rays. Furthermore a scratch will likely only affect the reflectivity of the (that tiny part) of the mirror.

A fibre blown to underside of the groundglass/fresnel-lens assembly would in any case be more visible.