I was thinking something similar actually. I already have an op-amp based circuit I built for better audio output so I already have a panel on the back. If the sync solution was small enough, could just bust out a MIDI connector on the back and call it done.
That sucks about needing to crimp though...bleh...crimp terminals and some glue solves that problem though I guess.
Newer cables may be different, don't know. But size wise we'er talking about a dip 8, a dip 16 and a dip 18, xtal and a few caps and resistors; on a proto about 1.5 x 2.5" May be a little bigger on a PCB.
I built the V1 on a Dontronic's SimmStick 18pin PIC board I had on hand, it's 2x3.5" but wastes some space. And I did a Midi thru with a 74HC00, could just have the PIC echo any in coming byte for Midi Thru and loose the 'HC00.
Should fit in a NES, there is so much space in that case!
yogi