I must be blind too. I was going by the printed manual. I can not find it either.
3.2.6. Option -O

The -O option causes the letters following it to be passed to the assembler on its command line as options. Some pertinent PM options are listed in the following table:

PBP -ol filename

This example tells PBP to generate a filename.lst file after a successful compilation.

More than one -O option may be passed to the assembler at a time.

The PICmicro Macro Assembler's manual on disk contains more information on the assembler and its options.
If all else fails you could have your program rename the hex file when PM is finished.
Be another pain in the _ _ _ though.