I added a page to the wiki with information on using FTDI cables that have been modified to use a non-default (proprietary) ID such as those from RT Systems.
http://chirp.danplanet.com/projects/chirp/wiki/CableGuide_FTDI_OEM_Cables
It can still use some work. In particular, I wasn't able to use NQ1E's trick under Windows XP to get it to use the FTDI VCP driver without touching the .inf files. The procedure does work under Windows 7 and possibly other versions. (The difference is that while both Windows 7 and XP detect that the cable ID doesn't match what's in the .inf file, Windows XP won't let you update the driver, Windows 7 will warn you but let you click through to proceed.)
The only way I was able to get the RT Systems cable to show up as a COM port under Windows XP was to create a custom .inf file using FTDI's utility for making .infs. If anyone has a better way, I'd be interested in it.
While I was adding the link for this page, I couldn't resist doing some editing of the main cable guide page:
http://chirp.danplanet.com/projects/chirp/wiki/CableGuide
Comments and corrections welcome.