Greg,
what kind of CDI are you talking about? A standard good ol' fashioned CDI, or maybe a device such as a Dynon EFIS used in CDI/HSI mode?
The former is not supported as far as I know, unless someone would work on a hardware device to interface the instrument with the serial stream coming from the Garmin GPS.
The latter, instead, is very much supported today, with the latest firmware release coming from Dynon.
Take a look at this picture:
and picture just the right 1/3 on an EFIS D-10A. You feed the NMEA data stream into the D-10A, and you get an HSI based on the selected waypoint you have in your GPS. Not too different from what you already have on the screen of the GPS in all fairness, but you have a separate display device *and* a nice EFIS too.
Ciao, Luca
(not a Dynon user, just a lurker on their forum, happy to see what they're doing with to their customers with the same hardware and just free firmware updates)