Can you make a DCP player?

I could... however at the moment the available open-source JPEG2000 decoding software is too slow to decode DCPs in real-time on all but the very fastest computers.

Commercial DCP players have cleverer JPEG2000 decoders, the tricks of which have not made it into the open-source world (yet). This is why they can manage DCP playback on more moderate computers.

If and when the open-source JPEG2000 libraries get up to speed (or computers get fast enough!) I will almost certainly look into writing a DCP player (or adapting something like VLC to play them).