John
Well there is a problem. A plugin is only good after the audio has been recorded. If you want the audio compressed before it gets to the DAW you need hardware.
Actually you can patch around and accomplish.... noting the VSTi thread, which desires to be able to record a VSTi LIVE. Using a plugin would be basically the same principle. But, another reason why it would be nice to have track routing internally in Sonar so you could do such a thing. Obviously not using CPU hungry plugs to kill you on latency, but many plugs would certainly be usable for this. As a guitar player, using some of the FX plugs would be great to use live, NOT having to apply them after the track is recorded. However at 24 bit, the need for compression going in, is greatly reduced and I'd just add that type of processing after the fact.