Currently parameter tables do not support Global Variables. Even the pass-through solution in 21.2 doesn't work, because it sets the 'Data' node as a number but it should be a string. The best solution I've found is to use the standard "Set Node Value" template for the On Set field and manually change
reference.value = newValue;
reference.value = string.fromNum(newValue);
Of course, I still have to find the path to "MODEL:/Tools/GlobalVariables/MyGlobalVariable/Data" and paste that into the Reference field.