Using render targets to make part of a SWEP's model be see-through?

[PixelSoldier]

Hey there. Anyone who's used FA:S 2.0 or CW 2.0 probably knows what I'm talking about, scoped weapons that upon zooming in make part of the scope invisible, so you can see through it and shoot. In FA:S 2.0 the code for this is as follows(though I have no idea where I would put this, as DrawRenderTarget isn't an actual SWEP hook but was made so you wouldn't have to override the drawing hook it's used in):

[lua] local x, y, old, ang
local sight = surface.GetTextureID("sprites/scope_pso_illum")
local sight2 = surface.GetTextureID("sprites/scope_pso")
local lens = surface.GetTextureID("VGUI/fas2/lense")
local lensring = surface.GetTextureID("VGUI/fas2/lensring")
local cd, alpha = {}, 0.5
local Ini = true

function SWEP:DrawRenderTarget()
if self.AimPos != self.PSO1Pos then
return
end

if self.dt.Status == FAS_STAT_ADS and not self.Peeking then
alpha = math.Approach(alpha, 0, FrameTime() * 5)
else
alpha = math.Approach(alpha, 1, FrameTime() * 5)
end

x, y = ScrW(), ScrH()
old = render.GetRenderTarget()

ang = self.Wep:GetAttachment(self.Wep:LookupAttachment("muzzle")).Ang
ang:RotateAroundAxis(ang:Forward(), -90)

cd.angles = ang
cd.origin = self.Owner:GetShootPos()
cd.x = 0
cd.y = 0
cd.w = 512
cd.h = 512
cd.fov = 4
cd.drawviewmodel = false
cd.drawhud = false
render.SetRenderTarget(self.ScopeRT)
render.SetViewPort(0, 0, 512, 512)

if alpha < 1 or Ini then
render.RenderView(cd)
Ini = false
end

ang = self.Owner:EyeAngles()
ang.p = ang.p + self.BlendAng.x
ang.y = ang.y + self.BlendAng.y
ang.r = ang.r + self.BlendAng.z
ang = -ang:Forward()
local light = render.ComputeLighting(self.Owner:GetShootPos(), ang)

cam.Start2D()
surface.SetDrawColor(255, 255, 255, 255)
surface.SetTexture(lensring)
surface.DrawTexturedRect(0, 0, 512, 512)
surface.SetTexture(sight2)
surface.DrawTexturedRect(1, 1, 512, 512)
surface.SetDrawColor(255, 255, 255, 255)
surface.SetTexture(sight)
surface.DrawTexturedRect(0, 0, 512, 512)
surface.SetDrawColor(150 * light[1], 150 * light[2], 150 * light[3], 255 * alpha)
surface.SetTexture(lens)
surface.DrawTexturedRect(0, 0, 512, 512)
cam.End2D()

render.SetViewPort(0, 0, x, y)
render.SetRenderTarget(old)

if self.PSO1Glass then
self.PSO1Glass:SetTexture("$basetexture", self.ScopeRT)
end
end
end[/lua]

Which hook would this be used in? I want to take a similar approach with rendertargets for a CS:S AUG.