Ah-ha! Figured it out. In case anyone else could use the magic incantation (for the iogear device in particular):

screen -fn /dev/tty.usbserial0 115200,cs8,ixoff

-fn is a bit redundant with ixoff in there, but anyhoo - have fun.
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-- JD - SN# 040104008 (120GB Blue, Digital Out)