Hi,

In USB, host is always MASTER, and device is always SLAVE.

What it means is host always need to enquire the SLAVE about the required information.
Even to send some information to the device, the host first prepares the device for reception and then sends the data.

So, in this context.. In UserIO routine of the Bootloader you read the Key status, and store it in variable. From host send a command to enquire about the button status. Now, let the firmware send the response to the host when this request arrives.

Hope this helps.