Maybe you could do something like this:
You could create a panel with the needed resulotion (640x480) and put it on front of the 3d entity
BMAP* window_pan = "windows_pan.tga";
PANEL* windows_pan =
{
bmap = window_pan;
layer = 15;
pos_x = 0;
pos_y = 0;
flags = D3D | OVERLAY | VISIBLE;
}
ENTITY* 3d_object =
{
type = "sf_alien.mdl";
layer = 10;
view = camera;
x = 350;
y = 134;
z = 100;
flags = BRIGHT | TRANSLUCENT;
flags2 = VISIBLE;
}