If you look in the Jedi-SDL library, there's a Kernel effect system which I ported from some C code a long time ago, it's still there though.

I think that what you need for Glow is Gradual Outlines at progressively darker colours.

Basically, you create a surface to contain your glow,
fill it with black
and you draw an outline of your sprite on it. So your new surface contains a nice outline of your sprite.

Then using a slightly darker colour, on the same surface, you draw an outline around your previous outline..

(You'll have to check for the colour of the previous outline or you'll end up bleeding colour where you don't want it)

Continue doing this until you're using a dark enough colour.

then, Additive Blit your new glow to the screen.

The technique is quite processor intensive, so you'll probably want to cache your results.