There's an option in Terminal.app -> Preferences -> Settings -> Keyboard but it allows me only to set both option keys as meta keys. iTerm.app has such an option but don't like that application.
If you aren't totally set on using Terminal, I believe that iTerm2 has a setting to just use one of the option keys as meta. If you are, is there a setting to maybe use the Esc key as meta? I know that it's more convenient to use the option key, but the Esc key might be easier to set.
I have built a solution myself and it doesn't require abandoning Terminal.app in favour of iTerm 2.
It is a simple status bar app that runs in the background and rewrites all left Alt +
$KEY key events to two key events in rapid succession, Esc, then
$KEY; however, it only does this if Terminal.app is in focus.
You can find the source for the app here. You'll need Xcode to build it.