I have been working on the same thing for a few weeks, at this point I have not found a way to make this happen with a usb serial device. If this is a desk top computer than you can use a pci serial card and on laptops you can use PC Card or PCMCIA Card since this is a hardware device it is always present. The also have Express Card Serial cards. Older laptops have PCMCIA Slots, newer laptops have express card slots, there are a few laptops with both types of slots, so the age of the laptop will determine witch type of card you can use.
Make sure the laptop is turned off, the power cord is disconnected, and the battery has been removed before doing this. It is unlikely that there would be a problem but, I always error on the side of caution.
Here is a very simple way to find out witch type of card slot your laptop has. Place the pc card slot towards you, slide a credit card into the slot. If the credit card go's all the way in you have a PCMCIA Card Slot but, if the credit card only go's in half way than you have the Newer Express-card Slot.
I use the USB Serial Device almost daily so if I come up with a way to make that work I'll post an update here.
Best of Luck