We are only partially compatible with Target Disk Mode on Thunderbolt right now. There is a feature request in place to improve our support for Thunderbolt interface on a Mac. I know this is a probably a pain to require access to the USB superdrive to boot from a CD and decrypt. Hopefully they will get this much needed feature on our roadmap soon. I would highly suggest submitting an idea here, if you don't see one already, regarding supporting Thunderbolt interface via target disk mode. This feedback goes to our Product Management team for review on features our customers are asking for.
Go here:
https://www-secure.symantec.com/connect/node/add/idea
For Drive Encryption on a Mac select Security from the community (required) drop down menu. Then select PGP WDE for Mac.
Best of luck!
Edit May 2013: To be notified of when we support Target Disk Mode via Thunderbolt interface please subscribe to the following KB article:
http://www.symantec.com/docs/TECH201009
For a list of other supported recovery methods see the related articles section of TECH201009.