I finished up my serial port install on the WRT1900AC this evening, snapped a few pics. I'm waiting on higher access to the wiki so that I can upload images directly rather than linking them externally, but here's a sneak peak at the finished look:
I'm using a MAX3232 breakout board on the inside, along with a proper JST PH6 connector going to the built in header, and from there going to a Philmore 3.5mm jack. Externally, I found a DB9/3.5mm cable, these aren't common, but easy enough to find online, and run this to a standard USB/DB9 serial adapter.
As soon as I can upload images into the wiki, I'll put a full writeup there with a few more pics.