TB54 based on your description, I feel like you have a need that is different from the original post in this discussion.
Are you referring to setting a new homepage, or redirecting to a page?
Which page provided by which extension?
My comment from 2020 doesn't require access or modification to any server file, you can perform a REST API request from the browser console to change the Flarum homepage to another page provided by an extension even if that extension didn't list the page as an option in the admin panel. But this shouldn't be needed, extensions like FoF Pages already provide the control for this in the admin panel. If it's missing an extension author can add this setting very easily.
If you are trying to alter the built-in Flarum homepage through direct DOM manipulation, it shouldn't require any server-side change either. You will likely need to hook into Mithril lifecycle hooks though. This can be achieved even from code added manually to header/footer. It's easier when compiled with Flarum's webpack, but the code can also be written by hand, there are just some very long imports to write manually.
