This is a fantastic plugin.
The well-known 'Maintenance Mode' plugin no longer works (for me at least...) and I looked for hours for an alternative, and found 'Wet Maintenance'. It's great. Allowed me to do exactly what I wanted.
You can pretty much insert your own HTML into the plugin's PHP file and create your own splash 'Under Maintenance' page, which is exactly what I wanted to be able to do.
Thanks heaps! :D