In general if you want to apply global variables to tablets, you are going to have to connect to the tablet directly. There are a few ways to do that in the operator, but we recommend that you use vtctlclient ExecuteFetchAsDba.
For example if you want to switch sync_binlog off on a tablet temporarily you would performing the following:
$ vtctlclient -server localhost:15999 ExecuteFetchAsDba zone1-0000000100 "set global sync_binlog=0"
This would show the following result after checking the variable:
$ vtctlclient -server localhost:15999 ExecuteFetchAsDba zone1-0000000100 "show variables like 'sync_binlog'"
+---------------+-------+
| Variable_name | Value |
+---------------+-------+
| sync_binlog | 0 |
+---------------+-------+