Extension Attribute for Apple Remote Desktop Client version?

If you're talking about the ARDAgent version, the ARDAgent itself is located in /System/Library/CoreServices/RemoteManagement/ARDAgent.app

So...

#!/bin/sh

echo "<result>$(defaults read /System/Library/CoreServices/RemoteManagement/ARDAgent.app/Contents/Info.plist CFBundleShortVersionString)</result>"

While you could do something like a test to make sure the ARDAgent is there first, its nearly always there since its a default part of any OS image, unless you've specifically ripped it out, so I don't think a test for this would be necessary.

I'm actually surprised there isn't a built in EA in the JSS templates for this. I see the ARD fields ones, but not anything like the above.