If it needs something from 10.5.4 maybe you could install the 10.5.4 SDK from an older version of Xcode alongside your existing install?

I don't know if it will go down in flames though but if you're going to downgrade anyway maybe it's worth a try.

I have 3.2.1 installed here on Snow Leopard but I see the 10.5.4 SDK is installed when checking with System Profiler. Unfortunately I don't remember how it got there but I know it had something to do with compiling the SDL Library.