How do I force MSBuild to compile for 32-bit mode?

According to MSDN, you’re doing the right thing. Looks like /p:Platform=x86, but actually, maybe it’s /p:PlatformTarget=x86.

Try to just invoke MSBuild directly with that parameter (make sure it’s not an issue with your NAnt file. Look at the build output for the right build configuration (Debug / Release).