To have the ability to set selected RMOPARMs to another value while Deliver is running (using "/F rmostc,SET initparm=operand"), you first need to set the RMOPARM SETCMD to SETCMD=YES (from the default of SETCMD=NO).
If you change the parameter, please recycle the RMOSTC task (stopping only needed with "/P rmostc"), for the change to be brought in with the next task start.
Please consult the Deliver Reference Guide for more information on which parameters can be changed while the Deliver task is running.