The Dreambox PVR600 has the ability to be controlled over the network and to stream TV.
But LinuxMCE has no support for Dreambox.
If you can find a Neutrino image for the PVR600, then it is possible to add it to Mythtv and use it as if it was an internal TV adapter. I'm not sure how it works or if it supports the PVR600, but if you know how to flash your Dreambox it's worth a try...
The best option for now, would be to use the Dreambox with one of supported PVR cards to capture the stream, and use a USBUIRT to control it. With that you can watch your Dreambox from any MD in the house and control it. Since you only have one Dreambox, all MD's can watch the same channel but not different channels.