Through a bug I discovered which makes the thermal vision work. to have custom models show up on the scope all you need is to simply add a texture or mask to the alpha channel of the diffuse.

By default L4D2's common infected have a texture in the alpha channel of their "diffuse" for use in the Infected shader. So by default, Vanilla commons will always show up highlighted on the scope. But Special Infected and Survivors normally won't since they have no texture in the alpha channel.

simply add one, could be a full black color in the alpha channel and it'll do the job.