I know you can do this in CSGO and even L4D2. As long as the value of the "basetexture" item in the vmt file of the lens is changed to _rt_SmallFB1 or _rt_FullFrameFB (it seems that _rt_Camera will also work), the lens can map the player's screen (of course, the display of the v_model will be ignored), and even Supports dynamic in-mirror magnification effect.
I wonder if it's possible with CS Source to do this? Even if the CSS shader doesn't support it well, can a plugin help achieve this effect?