The DEC drivers just left off the last 72 sectors. There was no provision using them for block replacement.
Minor update -- no dynamic replacement provision. There was a standard alone utility called BAD144 that implemented DEC Standard 144 (Bad Blocks). What would happen is that the format was messed with the point a bad block at on the reserved sectors.
This scheme caused nasty performance issues because seek schedualing was defeated; and since UNIX did not originally obey DEC STD144, the standard UNIX scheme in those days was buy 'perfect' media. BTW: the Ultrix team - Fred Canter IIRC -- added support for BAD144, and I think DEC gave that code to BSD evetually.
Clem