You could change the file that is loaded instead of the bp-maintenance.php file, but this would make something very simple, then very complicated. I added all the basic coding to that file so that it could be customized very quickly and easily. It contains inline CSS and is a self contained file / template with its own <html> and <head> tags.
I will never be changing the bp-maintenance.php file in any future version updates of BPS or changing the coding in BPS to use another file or handle this file any differently so as long as you have a copy of your customized bp-maintenance.php file downloaded to your computer then when you upgrade BPS you can just upload your customized bp-maintenance.php file and overwrite the default bp-maintenance.php file that comes with BPS.
I think using the existing bp-maintenance.php file is the simplest way to go without having to do any additional coding modifications to BPS. And the bp-maintenance.php file is designed to work as a stand alone file / template. In other words a drag and drop file. Thanks.