Of course, have been using different host names since the functionality was added. If you read the last commit notes of libshairplay, the mac address issue is described, I remember seeing there should be now possible to pass a different mac as a command line parameter to the executable. As far as I know, kodi makes use of the library, so not sure what it takes to get it implemented as a setting.
Edit: Actually the latest commit of libshairplay just fixes the formatting in its readme file, nevertheless the parameter there is the one I was mentioning:
- --hwaddr=address Sets the MAC address, useful if running multuple instances
Shairplay advertises over mDNS in the form 485D607CEE22@Dining Room Speakers where 485D607CEE22 is a MAC address. This is hardcoded to a nonsense value, which is not a problem so long as it's unique. However, if you run multiple Shairplay instances, iOS devices will only recognize one of them. To fix this, supply the --hwaddr option with different values on each device.