For anyone following along, I uninstalled and reinstalled SmarterMail (per a normal update process) to try and answer my own question about whether that companyinfo.xml would get overwritten wiping out my changes. The good news here is NO, that file did not get overwritten, or if it did, it retained the custom changes in addition to anything the installation put into that file. I say that because the DATE/TIME MODIFIED on that file updated to the new installation time, so it tells me SOMETHING read/wrote that file and saved it.
However, the changes to the file still do not fill in the login form.
This included a system reboot during the uninstall / reinstall.
Looking for other options, anybody ?