Currently there's not a place to do this from the Web GUI. You'll have to make the change manually from the CLI.
# dbc 'update ap set planned_maintenanced_mode=1 where id in (a,b,c);'
where a,b,c = the id numbers of the devices you want to put into maintenance mode
To find all your controller ids:
# dbc 'select id from ap where controls_aps=1;'
To do both parts in a single command:
# dbc 'update ap set planned_maintenance_mode=1 where id in (select id from ap where controls_aps=1);'
Then to revert, you can do the same command but toggle planned_maintenance_mode=0.
Or if no devices should be in planned maintenance mode:
# dbc 'update ap set planned_maintenance_mode=0;'