You probably want a reverse proxy. Apache can play this role:
If you have something like Varnish in front, that could serve as a reverse proxy instead.
Either way, the "http.path" configuration parameter may be useful:
For example, if the reverse proxy exposes the application at /foo, this config setting can tell Play to prepend /foo to the URLs it constructs so that the URLs the browser receives will actually work.
In the unlikely event that Play can use a dedicated IP address and/or port, you could also just expose it to the internet directly, without any reverse proxy in front.
-Matt