Jud,
Any Plaback Drive implies his own sound anyway, and trying it on the SSD might just as well work out for the better as for the worse. It is just for you to try out.
What's important though is the understanding that it is actually unrelated to your question. So, you can create a Playback Drive anywhere, as long as it is just that.
So ... create a partition on the SSD for it and have it a go.
But :
I think chances are fair it won't work at all. That is, if I understand it correctly that this is actually your OSX boot disk. I have, by accident, a same situation here, but it really needed tweaking to get it done. This is about
a. the Win partion on an OSX disk while the OSX disk won't let itself expand after all has been setup with Bootcamp (and sure not from the Windows side);
b. you can get it done through exFAT formatting IIRC which is compatible with Playback Drive usage.
I'll see whether I can't shrink the OSX partition a bit (there's quite a bit of unused room there, but I'd like to leave plenty of spare, which is why I asked the 10GB question) and install a new NTFS partition in the spare space. Otherwise may use exFAT, though I could also experiment with making it another OSX partition - I've got a proggy installed from Paragon that allows Win7 RW access to OSX volumes, and perhaps it would allow XXHE to see an OSX partition as a viable playback drive?
PPS: What about a small USB hub ? that should work.
USB hub for better sound with USB2? B/c my impression at least was the 2009 MBP has no ports that will do USB3 on a hardware level. Or did you mean something else?