I have recently installed Raspbmc on my R-Pi and I am using an ethernet cable via my laptop to connect to the internet, I now have a local IP address (10.42.0.255). Is there any way to still enable airplay by re-routing it through my laptop? I've looked around for an answer but I simply can't find one.
If you can set up your laptop as a wifi access point and then connect your Apple devices to that then airplay should work. You'll need to consult the documentation for whatever operating system you run on the laptop to find out how to run it as an access point. You'll need something like DHCP working (on the laptop) to automatically configure any wifi devices for you -- instructions for setting it up as an access point should help you with that.
The more common configuration is to use a dedicated access point. I guess you have other reasons for not wanting to do that.