I have managed to get a working example of razor/blazor working in the backoffice, based on everything that has been suggested so far and some google research. I Registered an extension as you would normally, but the JS template has an iframe that loads the razor page. Then I added a start up check on the razor page to ensure there is a referrer and it matches the site. I’m not sure it’s a clean or a secure solution but it does work.
Honestly, as I’m a new to Umbraco, right now I have no use case to need it, but I think it will be potentially useful.