mc.mcJogSetRate is covered in the Mach4CoreAPI help file. That is what you will need to use. I'm pretty sure changing the value of a DRO (via script) displaying current jog rate will not change jog rates, only the DRO. Your interaction with the DRO is different than clicking in the DRO and entering a value. Therefore different events will or more accurately will not get triggered.
Two things to try.
It may work if you add script including mc.mcJogSetRate in the droJogSpeed "on update" script.
In the script you are changing the DRO value........ instead of changing DRO value set the jog rate using mc.mcJogSetRate.