Not that I know of, I'm afraid. You could try capturing the audio through the PC's generic record utility through line in.
The trouble with OL CS 5 is that it's VERY fussy about datarate. Whilst CS4 was very happy to record onto a 7,200 HDD, CS5 causes frequent bottlenecks. I had to buy an e-sata raid drive running Raid-0 to eliminate this.
My point being, I don't know whether OL would manage to run in unison with anothr application. In any case, you would have to re-sync.
You will have to record audio separately on pc using Audition or some other soft, and video on Onlocation. if you what to go this route.
However if you would use your mike and little audio switch board that would give you line out, then you can switch your camera XLRS to line In and use it
which will give you much cleaner sound input then mike.
You also asked about separately Audio recorded sync. There is a solution to that as well. Check out this software
it offers a sync of an audio and as a little small app it works very good on any platform and for any editing application. It would be very nice if Adobe adapts this feature to their package of premiere and Audition.
Also if you record one or more camera and want external audio recorded at the same time check this one out
Something like that would nice to incorporate into OnLocation to make it top notch soft.