To turn a library into a bundle I think the following should work:
To the extra linker flags add:
To the Custom Xcode flags of each configuration add:
This isn't perfect as the xcode project will still show the icon as a library, and it won't link to the correct product. However I think you could approach it from a different angle, given that a plug-in is a bundle you could start with an audio plug-in and work backwords. Disable all the plug-in formats and use the Custom Xcode flags to set things to a more normal state, i.e. no need for all the rez stuff etc. Have a browse through the build settings in Xcode to see what you don't need.