Re:
Pretty sure that hole was for a cable hanger to keep the gear and brake cables away from the rear shock. One of my Marins that runs a 184mm shock has the same hole (although I thought it was smaller than 8mm) but the other that runs a shorter (170mm I think) does not. They are both from around 2000/2001 and both have the 27mm seat post and ovation tubing.
Interested to hear how you removed the brake bosses, it is something I have considered in the past.