I don't have any experience with USB to RS232 converters with Linux. But I've succesfully tested an ACM compatible (very important) USB modem with Slackware a few months ago. So, if you want to use the serial port to connect with an analog modem you can look for and ACM compatible USB modem instead.
There's just one big IF, which is support for ACM modems in the router firmware. I suppose its not included by default.
