Thank you for your post.
I'm not sure how you are specifying a Mac export. It would be the same for Windows.
If you are only hosting on a Mac and not using Get (HostApplication) and Get (SystemPlatform), then you should consider this.
Have a Startup script that checks HostApplicationVersion. If the file is hosted, then it will be blank. At that point, you can populate a field with Get (SystemPlatform). Then, the guests who access the file can check the field to determine what export to use. A sample script could be:
If [ IsEmpty ( Get ( HostApplicationVersion ) ) ]
Set Field [ <designated field> ; Get ( SystemPlatform ) ]
Click a button on the layout to execute a script on the hosted file, which then checks for the platform and decides which export script to use.
Let me know if you need additional clarification.