Jim, I used to help moderate an auto forum a few years ago, but I don't want to come off as an expert, because I'm not. I'd have a hard time finding my way through PHP and MySQL settings now. It's been a few years.
Also, my hair is grayer now and I use it for a good excuse to forget things.
Other forums that I frequent have an internal messaging system, which seems to emulate internal email. There is a "PM" (or similar) button icon next to a user's name/icon, which will
open a dialog box to send a private text message to that user. The text box behaves pretty much like the text boxes we use to post replies in threads, but goes to that user only.
Messages also get stored much the same way our emails do, with "inbox, sent, deleted", etc. I don't know how much server space HW has, so I can envision that as being a sticking point. They might not want to enable PMs.
Doug