Sometimes, museums just pop up in front of you.
Somehow I wasn't aware of this museum, located almost right next to the Korean National Police Heritage Museum. It opened in 2002, but it either wasn't listed or I overlooked it in my guide book while picking out places to visit in Seoul. Yes, I saw the Agricultural Museum and the Police Museum, but not the local history museum.
Not everything has explanations in English, but almost everything in the permanent exhibition has English titles, as you can see from this collection of smaller dioramas relating to the occupation period:
Admission is free and the museum opens at 9:00 daily except Monday. It closes at 20:00 on weekdays, and 19:00 on weekends and holidays (18:00 on weekends and holidays November-February).
Access is fairly easy from the subway Line 5. The closest stop is Gwanghwmun; take Exit 7 and walk straight until you see the museum above. From Line 5's Seodaemun Station, take Exit 4 and walk straight as well; you'll pass the police museum on this route. Lines 1, 2, and 3 also stop nearby. The museum's website has pretty good English information.