I know many have complained in the past about the changes to the Pi3 that made it hard to blink the activity LED and completely impossible to control the power LED.
I'm not sure if anyone else noticed but when someone asked the other day about the list of mailbox property tags looking at the latest copy of the list in the Linux source made me wonder what some new tags like RPI_FIRMWARE_GET_GPIO_STATE and RPI_FIRMWARE_SET_GPIO_CONFIG actually do.
It turns out the firmware was recently expanded to include a new set of GPIO control functions that allow almost complete access to the GPIO expander that is connected to the GPU. After looking through the changes to the Linux source and a small bit of experimentation we have a working driver that can blink the red LED, here's a quick video on Twitter.
And here's the source to our bare metal driver, yes I know it's in Pascal but you can cope with that I'm sure