Per Bull Holmen

local function makeProxyWrapper( proxy, key )
    local func = proxy._file[ key ]       -- find the value
    if( type( func ) == "function" ) then -- if it really is a function
        return function( proxy, ... )     -- then we return a wrapper instead
                    return func( proxy._file, ... )
               end
    end
    return func                           -- else we return the value unchanged
end

function newFileProxy( name, mode )       -- this will be the constructor
    local proxy = { _file = io.open ( name , mode ) } -- This is the file proxy table
    local meta = { __index = makeProxyWrapper }
    setmetatable( proxy, meta )
    return proxy
end

fproxy = newFileProxy( "/path/to/lua_test_file", "r" )

number, line, fifteen_chars, rest = fproxy:read( "*n", "*l", 15, "*a" )

print( "Number: " .. number )
print( "Line: " .. line )
print( "15: " .. fifteen_chars )
print( "Rest: " .. rest )