Hi Nigel,
Thank you for your interest! We got a few requests to make a module for the BDP-105 as well, but I am sorry to say that we don't plan to make one due to economical inefficiency of doing so. The BDP-103 and BDP-105 are identical in terms of audio and video playback capabilities (the sample applies to D versions). The "only" difference is USB input and analogue audio board (expensive DACs). You pay 35-40% more for BDP-105 just because of higher quality analogue audio board that you would throw away if you were to replace it with a digital output board like the VanityHD.
Of course you can use 2 DAC or digital amps for 4.0 speaker configuration as you describe. For instance multiple PS Audio DS DACs works really well with the Vanity HD module. Oppo's speaker setup is pretty comprehensive so you should be happy with that.
Should you have more questions, don't hesitate to ask.
Regards,
Pavel