Summer Sonic posted them just now and show BABYMETAL playing between 18:50-19:50 on Friday, August 16th in Osaka, and 19:00-20:00 in Tokyo the next day.
BABYMETAL almost always play 45 minutes, even when they are given 1 hour on the timetable. Here are the full timetables:
Yes, they again play after Bring Me The Horizon, their “festival friends” (who they saw again this month at Glastonbury), so we might see some pictures again.
As for the stage, a month ago, it was announced that BABYMETAL would headline the Mountain Stage at both venues. The Mountain Stage is the 2nd largest stage.
Tickets for Tokyo (Day 2) sold out 2 months ago, and for Osaka (Day 1) a month ago.
They also released the maps. The Mountain Stage in Tokyo is inside the convention center:
Here is BABYMETAL playing at the Mountain stage in Tokyo and Osaka in 2015. Do note the Mountain Stage has moved since BABYMETAL played there, it used to be where the Ocean Stage is now.
And for comparison, this was BABYMETAL playing at the Marine stage (headline stage) in Tokyo in 2017 (it’s a stadium) and Osaka’s Ocean stage (also headline stage):
This is the festival t-shirt that includes BABYMETAL, in black and white (note that BABYMETAL will very likely sell their own tee/s):
They also announced the full (and final) lineup: