Limit, remove or mask inward brightness on mesh outline

I am looking into creating soft outlines for meshes (a good example is shown here). However, the spiral blur effect mentioned in the blog has several limitations, is hard to work with due to being written in HLSL, and most importantly, has an extremely detrimental effect on performance when used with settings that provide decent visuals. I have no idea how Valve managed to make their soft outlines so cheap in their Source engine, but we can only work with the tools at our disposal.

So, instead, I was thinking whether it would be possible to somehow limit the “inward” brightness of the emissive color, thus simulating the blur/softness effect on the outward outline, at a very cheap cost.

Here’s what I’m talking about. Note that “brightness” in this context is just the color multiplied by a float, and the output goes straight into the “emissive color” node.

This is brightness set to 50:

This is brightness set to 500 (note the bright color affecting the mesh “inwards”):

This is roughly what I’m trying to achieve (excuse my poor photoshop):

I hope this makes sense.

Any help would be greatly appreciated.