I looked at reverse proxying first, however, we needed something we could change often without getting a new certificate.
As soon as I that adding a new domain would require a new certificate, well that ruled it pretty much out. Even a 3 month free one takes time!!! Of course you could use the one generated, but... that stupid RED warning!
Also, it is easy to create and save certs in ispConfig. Adding subjectAltname lines to the CSR seemed to mean manual work... and I hate work! I did not see any way to do that within ispConfig and I am never eager to make simple things more complex.
I was pretty happy when I realized that an SSL site connection would follow links on the same server. That makes it SO easy after you have a working, certified SSL cert installed. Just create the link! Nothing to edit, and it seems to work exactly like a site installed SSL cert, except for the URL of course...