How can I build a game with Unity 4.3 that runs on OS X 10.5 (Intel)?

My game (Mini Metro, standalone alpha build available here: http://dinopoloclub.com/minimetro/builds/alpha8/MiniMetro-alpha8-osx.zip) is built with Unity 4.3.2f1 and works fine in OS X 10.6+. But nobody's able to play it in OS X 10.5, even with an Intel processor. The command-line output is generally something like this:

 dyld: Library not loaded: @executable_path/../Frameworks/MonoEmbedRuntime/osx/libmono.0.dylib
   Referenced from: /Users/[x]/Desktop/MiniMetro-alpha8-osx.app/Contents/MacOS/Mini Metro
   Reason: no suitable image found.  Did find:
     /Users/[x]/Desktop/MiniMetro-alpha8-osx.app/Contents/MacOS/../Frameworks/MonoEmbedRuntime/osx/libmono.0.dylib: unknown required load command 0x80000022
     /Users/[x]/Desktop/MiniMetro-alpha8-osx.app/Contents/MacOS/../Frameworks/MonoEmbedRuntime/osx/libmono.0.dylib: unknown required load command 0x80000022
 Trace/BPT trap
 logout

The 'unknown required load command 0x80000022' error appears to mean the libmono dylib isn't compatible with OS X 10.5, but the Unity system requirements (http://unity3d.com/unity/system-requirements) are quite clear that 10.5 is supported as a runtime environment.

Is there something extra I have to do to get our builds working on 10.5, or do I have to state that it's 10.6+ only?

Thank you!