HID keyboard with backchannel

Hi,

I'm trying to create a HID keyboard based on HIDKeys with a backchannel for sending HID feature reports. I'm having trouble setting up the descriptor for this. Can anybody suggest where to look for something like this? I just need to be able to use this backchannel to send one feature report to issue a reboot of the device to bootloader for configuration and easy firmware upgrades. The most important bit is that communication with the device via this back channel has to work on all platforms: Windows, Linux and OSX.

Any suggestions on which direction I have to go would be highly appreciated.

Regards,
Arvydas