Since the drive encryption does not work in Xenapp, Symantec Encryption Desktop can't be listed as supported for that platform. From what I have seen, everything works other than the drive encryption, but since one part fails, it has to be labeled unsupported. As such, none of the components get tested in Xenapp, so it is a "use at your own risk" scenario.
If you are using an encryption management server, you can set the policy to hide the PGPtray icon in the system tray. Otherwise on standalone applications it is under Tools>Options, on the General tab. Uncheck the box for "Show PGP Icon in the Windows System Tray".
To get Outlook to function properly with the product, the PGPtray.exe process must be running. The best scenario for this would be if you can get the PGPtray application to run on startup. That would best mimic the use case in a standard, non-virtualized environment. I believe Citrix has tols for that.
Otherwise, you could just include a 'pause' in your launch script, to give PGPtray.exe a chance to load before Outlook pops up. My system takes around 3 seconds before it fully loads PGPtray. I would imagine that if you set it for 5-7 second pause, it should work. I would recommend using something like:
timeout /t 5 /nobreak
The nobreak flag forces the user to press Ctrl-C to quit, or to wait for the specified time. You could include some output to let them know something like "Preparing Outlook for email encryption" or whatever other text you might want the user to see.