You just need to work a little harder.

\startbuffer[1]
This is a test
\stopbuffer

\startbuffer[2]
Another test
\stopbuffer

\startbuffer[3]
And again
\stopbuffer

\startluacode
 userdata = userdata or {}
 function userdata.shift(i)
   return "(" .. i .. "cm ," .. i .. "cm )"
 end

 function userdata.buffers(i)
   local idx = string.format("%s",i)
   local b = buffers.data[idx]
   if b then
     if type(b) == "table" then
       return table.concat(b)
     else
       return b
     end
   else
     return "unknown"
   end
 end

 function userdata.draw_buffer(i)
   return "draw btex " .. userdata.buffers(i) .. " etex  shifted ".. userdata.shift(i) .. ";"
 end
\stopluacode

\starttext


\startluacode

 context.startMPpage({offset = "1ex", width = [[\paperwidth]], height = [[\paperheight]]})
   for i = 1,3 do
     context(userdata.draw_buffer(i))
   end
 context.stopMPpage()
\stopluacode

\stoptext