Why not make a LFE send per track..? Just like the center send..? You can always make an individual LFE track if you want to like it's working now..! But a LFE send per track is much more usefull I think than you can even automate the send slider..! :-)
You can already do this:
-Add a Sub bus to the mixer.
-Set its Surround panner to LFE mode
- Send any track you want to this SubBus using the send assignements on the multifunction fader.