R – Force a Mac OS X 32/64-bit universal binary to run 32-bit on an old OS version only

I want to ship a single 32/64-bit universal binary of my desktop Mac OS X Cocoa app. I don't want to ship two versions.

I want it to run as a 64-bit process on Snow Leopard.

However, if the app is launched on Leopard, I want to always force it to run as a 32-bit process.

NOTE: I know that in the Finder 'Get Info' window you can click the 'Open in 32-bit mode' checkbox to achieve this. That's nice, but what I really want is a way to automatically, always force this behavior on Leopard only. On Snow Leopard the app should run 64-bit unless the user wants to change the setting mentioned above.

Can this be done? If so, how? thx.