I'm new to this forum, otherwise I would've answered earlier!
I'm an avid Muzys user, and yes I have run Live and Muzys together. In fact, I just did a gig (short one albeit - 1 hour) running both simultaneously.
I have done some experimentation using rewire, and the main limitation is that Live must be run as a slave from Muzys. I haven't found a way around this yet. But, like I said, I haven't done all that much with rewire. So, it's still possible that Live might be able to run Muzys as a slave. In any case, Muzys is another special app, like Live, that deserves you're curiosity. It is also know for it's stability - in fact, the installer touches ***NO*** system files on either Mac or PC. (But just so you OSX'rs know, Muzys too, is behind in thier support for OSX. But, this is a major feature of the next version - as voted by it's user base! A very proactive user base that the devlopment team at Muzys really cares about.)
I'm a bit spoiled with a 3.06ghz P4 laptop with 1gb ram running WinXP, but the icing on the cake is the Echo Audio Layla Laptop with it's multi-client drivers. I can run Live along with about any other app I want. I'm also using a nice VST host/rack prog from Xlutop called Chainer (similar to RT Player). Chainer's pretty nice (and affordable!) and you can also route other analog signals through it. I have actually run Live, Muzys and Chainer all at the same time with no problems. Tracks playing back from Live and/or Muzys (Nice way of segueing between entire *songs*), soft synths being played within Muzys, and an aux send from a vocal mic running through OHMboyz FX in Chainer. All this with about 6ms latency. Pretty darn sweet.
I understand RME has multi-client drivers too, and with the right DAW, you should be able to do similar things. I decided to go with the Layla in the end, because of Echo's impeccable reputation of quality hardware and programming (drivers, baby!)
Muzys' forum is over at KVR.
http://www.kvr-vst.com/forum/viewforum.php?f=21
~Exit Zero