I do not have a USB serial device, but there must be a way to find the real ports using the HAL libraries directly:
====================================================================
#! /usr/bin/env bash
#
# Uses HAL to find existing serial hardware
#
for sport in $(hal-find-by-capability --capability serial) ; do
hal-get-property --udi "${sport}" --key serial.device
done
====================================================================
The posted python-dbus code nor this sh script lists the bluetooth /dev/rfcomm* devices, so it is not the best solution.
Note that on other unix platforms, the serial ports are not named ttyS? and even in linux, some serial cards allow you to name the devices. Assuming a pattern in the serial devices names is wrong.