Building a device to multiplex the serial port should be trivial. The hard part (which really isn't all that hard) would be getting the player to decode a header byte and handle the data appropriately. The only issue that I can think of is if two devices want to communcate at the same time to the player, one of them is going to not get their data acroos unless they are waiting for a "go ahead and give me your data" message from the multiplexor. This is doable, provided that the devices that are sharing the serial port can be custom programmed to wait until its their turn to transfer data.

The control would be very simple in that case:
1) Send "its your turn" signal to device 1
2) Recieve data from device 1
3) Transmit data that was recieved from device 1 to the player
... repeat for every device

Actually constructing the HW and the PIC SW would take me about 15 minutes to design, but due to the simultanious transfer problem, I haven't found it to be a particualrly useful application.