indicator-multiload from the default repository is as close to this feature as I can find. It displays a small rectangle in the notification area. By default it shows the CPU usage. You can change the preference and select which usage from 6 choices including Harddisk. You can simply remove the check mark from Load and checkmark Harddisk.
binary indicator criterion is a vertical line on or off.
I need this functionality because, while the monitors are on my desk, the computers are hidden, so I can't see the flashing lights on them. Also, like you, one of the computers is a mini-box, that doesn't have an harddisk LED.
Since this question was high on my google search for a solution, I'm sure it will benefit others looking for something similar.
Hope you can use it.