[semlar]

You can get the original metatable of the frame and set that as your new metatable's metatable, eg.

local f = CreateFrame('frame')
local mt = {
    foo = function()
        print('bar')
    end
}
setmetatable(f, { __index = setmetatable(mt, getmetatable(f)) })
f.foo()
print(f.SetBackdropColor)