[Fizzlemizz]

I was thinking something like:

local frame=CreateFrame("Frame")
frame:RegisterEvent("PLAYER_LOGIN")
frame:SetScript("OnEvent",function(self,event,...)
    local chat = DEFAULT_CHAT_FRAME
    local editBox = chat.editBox 
    editBox:ClearAllPoints()
    local x = chat:GetLeft()
    local y = chat:GetBottom() - editBox:GetHeight()
    editBox:SetPoint("BOTTOMLEFT", UIParent ,x ,y) 
    editBox:SetWidth(chat:GetWidth())
    editBox:SetMultiLine(true)
    local n = editBox:GetName()
    _G[n.."Left"]:Hide()
    _G[n.."Mid"]:Hide()
    _G[n.."Right"]:Hide()
    editBox.focusLeft:SetTexture(nil)
    editBox.focusMid:SetTexture(nil)
    editBox.focusRight:SetTexture(nil)
end)

The editbox artwork doesn't adjust so I've just hidden it (some of the artwork regions might not be valid in Classic)