Those are really server options. Rackup isn't really a framework. People often want to extend it for this purpose, but isnt designed in that way really.
What you could do would be to write your own binary, using rack::server and then grab options from there, but in that case you'd really be better off implementing options yourself so that you have more control and aren't reliant on APIs that no one really uses (and thus may be subject to change).
Racks server infrastructure has always been designed to be used programmatically for this reason.