Apache HTTP Server and Apache Tomcat are actually two separate servers. The .htaccess file is used by Apache HTTP Server and not applicable with Apache Tomcat as of the version that we are using.
As for maintenance and showing or redirecting to another page, that would normally depends on what is available on your environment and how you would like to approach it. Like if it is behind a load balancer or a web server, it can be temporarily directed to a simple server serving that maintenance announcement page.
Or If you think that you will need a longer time, i.e. 30 minutes or more, to conduct test first. You can use some lightweight HTTP server out there which temporarily will accept the requests and server them with a maintenance page announcement. You would need to change the port of the server of Canvas temporarily as two(2) servers on the same machine cannot have the same port.
Another alternative approach is to inform users before hand. You can incorporate some form of announcement corner or section on your Canvas application and use it for something else later too (i.e. start of Budgeting Cycle until MMM 20YY, Maintenance Works on MMM DD, YYYY from HH:MM to HH:MM, etc.), that is being pulled from TM1 for example.