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> but the first parameter to the C function that implements 
> methodOne is not the userdata, but the table. So the function 
> fails. It would work for userdata functions that don't expect 
> the userdata as the first parameter, but that is not the 
> usual situation. 

You could make it work if you don't mind changing the C functions that
implement the userdata. If they find a table as the first parameter on
the stack then they could query a known field in that table to see if
the actual userdata object is there. Perhaps __ud or something. Then you
could do:

local userdata = system.createUD()
local t = { __ud = userdata }
setmetatable(t, getmetatable(t.__ud))

-- call a userdata method via t

And it would work. This is not a general solution though, since it is
not always possible or desirable to modify the C code.