Unfortunately the XenForo 2 add-on upgrade system doesn't support this. The only way to do it would be to use a custom upgrade system like older versions that doesn't use the XenForo 2 add-on system. But the XenForo system supports most of the other features of the old system which is why we opted to move in the direction of using a native upgrader; for example, processing upgrades in batches and steps and native support for CLI upgrades. Also I believe that it's required to use the native upgrader by XenForo add-on guidelines. For now you can review the progress in the log files at internal_data/vw_logs/upgrade. In the browser side, you can see if it's making progress by watching the browser state. If it's refreshing every few seconds, then it's making progress. If it displays an error, then it's not making progress.
The XenForo 2 upgrade process view is generated by the admin template addon_run_step. The only variable that is scoped into that template that the add-on setup can manipulate is $params. But $params is not used in a place on the template that would allow for more explanation to appear. We could theoretically add a template modification for that template, but it would not benefit anyone in the following cases:
- Installing VaultWiki for the first time
- Upgrading VaultWiki from XF 1.x
- Upgrading VaultWiki from a version before the template modification was made