What is the procedure when a required A/D inspection via an AMOC has gone overdue?
Each AD is specific (and unique) to the procedures and compliance outlined in the AD. If the AD does not say otherwise, all that is needed is to comply with it (do the inspection), and then make an entry in the maintenance log book, and you will be good to go.

I have never seen an AD that addresses circumstances when not previously complied with. For example, a Cessna 172 requires an inspection of the horizontal stabilizer spar; it does not matter if no one has done it for 45yrs. All that is needed is to do the required inspection and it is good to go :)