does anyone know if I can connect a usb MIDI device (my drum machine) directly to a controller port and have it recognised within the likes of Guitar Hero?
A Generic USB MIDI device? No.
Xbox 360 games (like Guitar Hero and Rockband) actually receive input from their controllers are if they were standard controllers with a few modifications. By comparison a Drum Machine which outputs MIDI does not conform to the standards. More than likely the Xbox will not know how to handle the input and ignore it.
No you can't plug it directly in but if you buy the MIDI adapter for rock band:
you can use your drums with Rock Band 3. Quick links for the easy shopping,