Would it be possible to use Dynamo to detach a file from Central rather than having to close the file, and having to reopen and clicking the “Detach from Central”?
No, you can detach from Central only when you are opening. You could use Dynamo to open a different file, as detached and save as somewhere else. You could even use it to open the one in which you were currently, detach and save somewhere else. Still, it can only be done while opening.