Sidechaining question

I would like to activate a send from the Master bus and put it through the sidechain of the Pro Channel compressor that is also on the Master bus.  The reason to do this is that I'd like to then use the High Pass Filter to keep the comp from getting triggered so much by the low end.
 
There is no option to assign the Master send to the side chain.  I tried to work around it by assigning the send to a new bus and then assigning that bus to the side chain, but the option wasn't there again.  I assume that Sonar is trying to prevent a feedback loop, but there is no feedback loop when your just going into the sidechain!
 
Any help is appreciated,
Seth

It is indeed the potential for a feedback loop. By default sends are post fader and the effects bin is pre fader so if you did a send to the effect it would create feedback. In this circumstance SONAR is correctly blocking it.
However if the send is pre fader technically it should be ok. Today the code doesn't distinguish between pre and post fader sends as far as sidechaining goes so its blocked. We'll look into it for a future version.