hi,

if you don't want pixel / vertex shader, then a 3D texture will do the trick : on a 3D texture, store many layers from your sun when it's partially occluded throught the sun when it's entirely seen (with a big glow effect on the texture), and depending on how much of the sun is visible, set the depth value of your 3D texture (they use that for lights effects during the night, in car driving games)

but this effect won't overlap properly on your environment ... and to do that, you'll need a post processing pixel shader I fear ...
If you know the game "Guild Wars", they have a really cool effect like that near the teleporters : a sort of big like which glow, and overlap on the environment, but I never understood how do they do that.

Oh, and by the way, would you mind sharing a screenshot of your engine ?? I'm always curious to see what people can do, and this forum lacks a bit of screenshots ^^