I know this is a very late response to a dead thread. But, in case somebody else happens upon this thread...
I wonder if something based on FT232RQ would work (USB to TTL Serial conversion chip).
Products:
http://www.ftdichip.com/Documents/DataSheets/Modules/DS_TTL232R.pdf
http://www.ecrater.com/product.php?pid=1750401