Can I use SLIP on the same line as DMX?
While it SHOULD work, you would be at the mercy of the DMX implementation on the receivers, and there is all sorts of weird out there. In practise you need to be very conservative in what you generate if you want random DMX kit from anything other then first tier vendors to work properly. 

Personally I would use a couple of the unused pairs in your Cat 5 line for the talkback, keeping this separate seems more likely to beget reliability.