Drop this in a .LUA to have a frame in the center of the screen that you can click through. The final two functions called are what makes it work, theoretically they should work on any frame Blizzard or not.
Code:
local exampleFrame = CreateFrame("FRAME", nil, UIParent)
exampleFrame:SetWidth(100)
exampleFrame:SetHeight(100)
exampleFrame:SetPoint("CENTER", UIParent, "CENTER")
exampleFrame:SetBackdrop({bgFile = "Interface/Tooltips/UI-Tooltip-Background", edgeFile = "Interface/Tooltips/UI-Tooltip-Border", tile = true, tileSize = 16, edgeSize = 16, insets = { left = 4, right = 4, top = 4, bottom = 4 }});
exampleFrame:SetBackdropColor(0,0,0,1);
exampleFrame:SetFrameStrata(BACKGROUND)
exampleFrame:SetFrameLevel(0)