If you don’t mind spending a little money, and you have a fast computer, you should consider purchasing the private roku channel called Playon.
It’s not perfect (it is dependent on your computer and internet speed), but it will give you hundreds of options for watching cable and local channels on your tv with your roku player.
Within playon, there are other scripts made by users (almost all are free) that allow you to watch just about any tv show and movie ever made. The main plugins directly from Playon are all on-demand programming, but the scripts even have live tv options.
I purchased their lifetime license (right now the price is $39-49)—this is a one-time purchase with a 30-day, money-back guarantee.
Website with scripts and plugins that work within playon:
http://playonscripts.com/
The plugins/scripts I use with playon are:
1channel (free) for movies and tv shows
projectfreetv (free) movies and tv shows
superusvox ($5 one-time fee) for live cable
However, I will have this thread bookmarked and I will visit your idea as time and money allows.
Thanks for the suggestion!