We believe the GM tag is tied to the 1st post, so it might be tougher to implement than more Co-GMs. 

Considering that there's only one real difference between GM and Co-GM at this point (appointing/removing Co-GMs), its not really a major distinction. 
a "better" solution would be to empower moderators to be able to change the author on first post, for circumstances like when the GM wants to quit but the rest of the players (and Co-GMs) want to continue.

Also, upping the roof on Co-GM count would be helpful and require less work.