Maybe would work if you set its "layer" lower than other entities.

In the manual...

Elements with higher layer value will be placed over elements with lower layer value. Elements with the same layer value are sorted due to the z buffer

entity shotgun_onscreen
{
type = <shotgun.mdl>;
layer = 2; // display above view entities with layer 1
flags = visible;// visible on screen from the start
view = CAMERA; // same camera parameters as the default view
albedo = 50; // gouraud shading from sun position
x = 100; // place 100 quants ahead of the view
y = -50; // 50 to the right
z = 0; // and center vertically
}


Another you could try would just be a flat transparent sprite infront of camera and use layers. Goodluck