I've had a really unsuccessful time in trying to get the USB HID to work on the Trinket Pro. From what I had seen, and read on the web, it should not have an easy task.
The problem that I encountered was that the Trinket bootloader(USBTiny) would start up, and it would be enumerated by the host system correctly. But, when the bootloader passed control to my sketch, the host would always failed to enumerate the Trinket as a Keyboard HID -- it would remain a USBTiny.
I spent several frustrating days trying to get it going. Out of frustration, I ended up modifying the Trinket.
It's a simple change. I added GPIO control of the USB pull-up resistor. I now works consistently.
The change which I made is to allow the sketch to pull the USB data low/high which causes the USB host to re-enumerate.
There might be a non-hardware solution, but for now, I am going to leave it alone. The current mod uses a diode, but just a wire may be sufficient.