Well, okay. In theory you can add an action hook to the 'init' action. In that function, you need to make a call to
ob_start();. This will enable the output buffering. You'll also have to hook the 'shutdown' action hook and call
ob_end_flush(); at that function, so the output will be sent at the end.
This will work, but it's not particularly advisable. It will force the webpage output to buffer in memory until all processing is complete. This is not a particularly big deal most of the time, but if you have every plugin doing it, then that's a lot of unnecessary memory usage. Using a flag is better.