My question:Would it have been possible to install the SSL certificates in the virtual machines?
YES. It's possibile to send Internet HTTPS traffic to an internal HTTPS service behind apache httpd as a reverse proxy.
You eventally need to install same SSL certificates (but you don't have to necessarily) on both the reverse proxy and the internal service, enable SSLProxyProtol on your VHs and send the traffic to HTTPS via your ProxyPass.