It turns out I had the incoming video jack soldered on incorrectly (that's what I get for not reading the datasheet). I removed it, and a few SMT resistors this morning to try to troubleshoot and found that absolutely no current was going through the video jack pins. At first I thought I had a defective port, then realized it was simply that the jack had a shunt pin in addition to a signal pin. Hehehehe.
I was hoping to post another picture in addition to some screenshots from my scope, but I ran out of time. I soldered up the little arduino board both to my VBI injector board and to 5 places on the dexter board (RX1, TX1, CSYNC, VSYNC, and FIELD). The gain on the VBI injector's amp possibly needs to be increased but my capture card is still decoding the picture properly so it is pretty close.
Assuming everything else works, all I need to do now is write the AVR program to inject the VBI.