As far as I know there is no way to force the game into widescreen but to be honest if you set you monitor to fill it looks pretty cool anyway (I couldn't play it at 4:3 as it looked squashed with black bars either side).
Yes, there is a way to play RS in many widescreen resolutions. I have tested this method and it works flawlessly for the game in my native 1050 x 1680 resolution.
You do have to start with a working no CD executable like it says.
I edited my exe to run 1050 x 1680 and saved it for future use so I don't have to do it again. The modified resolution will then show up in the graphics option in the game. Move the slider all the way to the right and you will see your custom widescreen size.