Bits pretty much covered all the points on why this is unbelieveable. GoT just doesn't have the wide appeal of LotR. As with the books, i'm already bored of the TV series... as are my friends who only discovered it through the TV series, already fed up with Martins killing a character every 5 minutes.
But yeah... Wrong time to do a tie in, especially with a TV show. Plus.. a boring one given pretty much every army would be exactly the same appart from the characters. The Lannisters etc would pretty much be your Space Marines, with only the Iron Islands etc having any kind of variation.