Apache2 Reverse Proxy to an end-point that requires BasicAuth but want to hide this from user

Basically my scenario is that I have an internal website that requires a SINGLE hard-coded username and password to access (and this can't be turned off, only changed). I am exposing this website through a reverse proxy for various reasons (hiding the port, simplifying url, simplifying NAT, etc).

However, what I would like to do is be able to use Apache to handle the authentication so that:

1. I don't have to give out single password to everyone
2. I can have multiple usernames and passwords using Apache's BasicAuth
3. For internal users, I don't have to prompt for a password

EDIT: Second part about richer authentication has been moved to new question

Here's more or less what I have now:

<VirtualHost *:80>
  ServerName sub.domain.com

  ProxyPass        / http://192.168.1.253:8080/endpoint
  ProxyPassReverse / http://192.168.1.253:8080/endpoint

  # The endpoint has a mandatory password that I want to avoid requiring users to type
  # I.e. something like this would be nice (but does not work)

  # ProxyPass        / http://username:password@192.168.1.253:8080/endpoint
  # ProxyPassReverse / http://username:password@192.168.1.253:8080/endpoint

  # Also need to be able to require a password to access proxy for people outside local subnet
  # However these passwords will be controlled by Apache using BasicAuth, not the ProxyPass endpoint

  # Ideas?
</VirtualHost>