I'm really just talking about the browser trusting the user cert itself. I've done the softcert thing myself before, I forget if it used commercial root CA or not but it did work.

I guess you could flag the leaf (user) cert as ultimately trusted and that should be fine, but if the browser doesn't see that trust notation, and does see an intermediate CA, it's going to try to pull that back to a trusted root.

One way or the other the user will have to fiddle with browser settings to make a CAC work, either to tell the browser to trust their cert explicitly, or to have the browser trust DoD certs.