Just completed my P5 build and have been thinking about the same thing. It would take some custom mounting work, but I believe a thin external optical drive could be mounted just above the power supply from the GPU tray, maybe using Velcro on the underside of the tray facing it out to the left for easy access with case cover on.
It could be connected to one of the USB ports of the mother board I/O ports. Also, for faster throughput you could put a USB 3,1 cable/connector directly to ur mobo if it supports one. Could paint the drive to match the case. What do you think?