Actually, its not that hard to get the Mac to release the serial device hardware so you can use it, assuming it uses a kernel extension (kext). All you have to do is unload the kext. This doesn not delete it, it just doesn't use it any more. All you have to do (and this may not be easy in some cases) is find the kext file that represents the serial driver (I actually examined the installer package and found the .kext file that was installed by it).

For example, to turn off the serial driver:

#!/bin/tcsh
sudo kextunload -v /System/Library/Extensions/FTDIUSBSerialDriver.kext
kextstat | grep SerialDriver

Once that is done, then you can select the serial device in the USB menu of Parallels and it should grab it just fine.

To turn the serial device driver back on (you should release the device from Parallels and pull it out):

#!/bin/tcsh
sudo kextload -v /System/Library/Extensions/FTDIUSBSerialDriver.kext
kextstat | grep SerialDriver

Now reinsert the serial device and the Mac should grab it.

I'm not saying that you shouldn't use the info in the original hint...this is just another option. Again your milage will vary by which device you're using. I can't remember the specific one that I have right now and I don't have it with me. Hope that helps someone.