CyrilLD over at XDA found the serial console header on the GW620! It's the 5 contacts in a row underneath the SIM card. The center one is the TX pin. This is a 3.3v serial port, so you'll need a level converter of some sort to connect it directly to a PC's serial port, but most USB-Serial adapters already operate at 3.3v so they should work fine.
I'm now trying to build a kernel (based on the default LG sources) that will output to that port during boot. If I can get boot messages on that serial port, then I should be able to duplicate the settings on the 2.6.29 kernel and hopefully get enough info to figure out where it's crashing. Finger's crossed!