At first glance it does seem like a good idea. When the roku sdk first came out I aquired it with much the same idea in mind. However, the roku does not give access to low level networking, does not support 1080, and does not support udp streaming. It was not designed with lan streaming in mind, and does not list mpeg2 as a supported codec. This may not be an
issue as the chip set says it supports mpeg2, however that still at best leaves
the other issues.
So you need something to at least convert the stream to tcp such as livestreamer, and for 1080 content a scaler. You may also need a transcoder to h.264. This is in addition to a web server based tuner controller for setting up the channels in a way that you can control from the available functions on the Roku. In any case, the best chance of it happening, would be on the roku side, not here. They would need to add support for udp streaming, bring out the mpeg2 support of the chipset, and add a way to tune the channels that can be accessed from the sdk (assuming they did not just bring out the channel at that point themeselves with their more full access to the box).
So, probably not going to happen, at least with the current sdk and roku boxes. I am currently leaning towards trying the Xtreamer for something like this, but it does not support netflix, sigh, always compromises.