Is there a way to tell Perforce to leave text files alone without setting the file-type as binary? Or barring that, is there a way to tell the Perforce client to diff binaries?
When adding text files that you want to be considered binary to Perforce, use the command p4 add with the -t option to override the type mapping table. See the documentation here. The example given there is p4 add -t binary file.pdf.