I had a more complicated use case and it required use ProxyPassMatch. This is a bit like this:
ProxyPassMatch ^/app(/(index.html)?)?$ !
RedirectMatch ^/app(/(index.html)?)?$ /path/to/login/page.html
ProxyPass /app/* http://remote-server/app
ProxyPassReverse /app/* http://remote-server/app
ProxyPassMatch, ProxyPass . , ProxyPassMatch $ .
RewriteMatch , Redirect -. .