Hi,
The "pinmux" command does not seem to be used for changing the SSS bit under the SIUL2.MSCRn register.
You should be able to "set" or "clear" a pin by using the "gpio" command, as shown below:

This command will set the OBE bit under the respective SIUL2.MSCRn register and set/clear the PDO_n bit under the SIUL2.GPDOn register.
This command will not change the SSS bit under the SIUL2.MSCRn. If you would like to modify this value you should be able to use the "mw.l" command, which will write a value of 32 bits to the defined address:

As seen on the above image, we changed the SSS bit for PA_06 (0x4009 C258) from 0x0 (GPIO[6]) to 0x2 (DSPI1_SOUT).
Please, let us know.