I've been a longtime Premiere user, but hadn't needed to delve deeper into Multicam sequences until a project I started today. I spent all morning and ran into the same limitation you described.
After tinkering around for a few hours, I've come up with a solution that may help out folks, even though this is an old thread back from June. Unfortunately it won't help out the Original Poster, but since this is one of the top google hits for this issue, hopefully anyone experiencing this problem will come by and see this!
Using screenshots and putting text together using Photoshop, you can view my workaround here:
The first step is creating your Multi-Cam sequence properly:
https://dl.dropbox.com/u/38225717/Multi-camCS6_1.png
The second step is duplicating the multi-cam sequence's audio onto separate tracks, then enabling Multi-Camera on the audio. You then switch each audio to the corresponding track number.
Hopefully that makes more sense in the image:
https://dl.dropbox.com/u/38225717/Multi-camCS6_2.png
It's not perfect by any means, and it's a little messy, but it'll get the job done. For me, it was a necessity as I am cutting scenes and may need to mute the boom track, adjust levels on a lav track, etc.
I hope my diagrams make sense; I sorta cobbled them together quick because I really wanted to share it with the Adobe community before heading back to work!