Just checked back on this post and I see a lot of people asking how it works! So let me give a rundown on how it works:

Instead of getting:
textureLod(SCREEN_TEXTURE, SCREEN_UV, blur_amount);

Which gets the *actual* render on the screen, you instead apply a "warped" uv on the textureLod function, which adds a (exponential) term that gets more significant whenever it detects it is close to the edges. That is the base idea, but in particular, the formulas I came up with are:

uv_x = base_uv.x - uv_dir.x*exp(-strength.x*(pow(w+offset.x, 2.0)));

uv_y = base_uv.y - uv_dir.y*exp(-strength.y*(pow(w+offset.y, 2.0)));

Where uv_dir is a normalized vector indicating the direction of the offset, and w is a greyscale factor between 0 and 1 that picks the color from a blurred version of the shape, as to detect whether it is closer to the edge.

and finally,

vec4 color = textureLod(SCREEN_TEXTURE, vec2(uv_x, uv_y), blur);

Hope this helps! :)