I seem to recall Boeing getting a special exemption not to have to install a 3rd overwing exit on the 737-900.
I believe you recall incorrectly.
IIRC, Boeing asked for an exemption so they could keep the single overwing exits on the -900. However, the JAA (or whoever made the call in Europe at the time) refused, supposedly due to Airbus lobbying. Therefore, Boeing spent a huge amount of $$ re-engineering the -900 for the dual overwing exits. The structural aspect was NOT straightforward...
AFAIK, there was never a requirement for a 3rd overwing exit on the -900.