I got it to work! ...and I'll post my findings here for posterity.
I installed the FTDI VCP driver then modified /System/Library/Extensions/FTDIUSBSerialDriver.kext/Contents/Info.plist to add a new Personality for my device using the ProductID and VendorID listed in the System Profiler. Loaded the kext, plugged in the device, and then i had /dev/tty.usbserial!
Useful links:
http://www.ftdichip.com/Drivers/VCP.htmhttp://www.ftdichip.com/Support/Documen ... Driver.pdfhttp://dfusion.com.au/wiki/tiki-index.p ... ver+on+Mac