The related idea of a tool that would allow PGN input to display a game board was brought up a while back here on meta. Incorporating the ability to input positions via FEN was mentioned in comments there as well. This would indeed be a very useful tool to have on the site. It would greatly streamline the inclusion of diagrams, and would greatly improve the inclusion of analysis/variations in posts. And the added uniformity and aesthetic appeal might help make the site more immediately attractive to new users too.
I don't know what the best way to go about implementing such a tool would be. One idea - I don't know how feasible this would be - is to incorporate use of the Chess Tempo PGN viewer on the site. I know that viewer is used by sites such as ChessVibes (see it in action in this post, for instance). Anyway, like I said, I don't know what the best way to do this would be, but you're right that it would be a fantastic addition to the site.