The question I have is can I just copy the App_offline.htm file I created for this, to the root level of the web app?
Yes, if you want to take a web application offline while you migrate a SQL Server database, in IIS and ASP.NET, you accomplish this by placing a file named App_offline.htm in the root folder of the IIS website or web application. The App_offline.htm file is a standard HTML file and will usually contain a simple message advising the user that the site is temporarily unavailable due to maintenance. While the App_offline.htm file exists in the root folder of the website, IIS will automatically redirect any requests to the file. When you've finished making updates, you remove the App_offline.htm file and the website resumes serving requests as usual.
More information about how to take Web Applications Offline you can refer to this link: taking-web-applications-offline-with-web-deploy.
If the answer is the right solution, please click "Accept Answer" and kindly upvote it. If you have extra questions about this answer, please click "Comment".
Note: Please follow the steps in our documentation to enable e-mail notifications if you want to receive the related email notification for this thread.