In an OnCommand... event handler, you can manually set the response's ResponseNo and ResponseText properties (in that order) to whatever values you want. The ResponseNo property setter will assign a default string to the ResponseText, based on HTTP standards, but you can just overwrite that value with your own string, eg:

AResponseInfo.ResponseNo := 308;
AResponseInfo.ResponseText := 'Permanent Redirect';

That being said, allowing the user to customize the default ResponseText strings is an interesting idea, but it will have to wait for a future release since that is a whole new feature set to add (new properties, design-time editing, etc).

But, in the meantime, I have updated the ResponseNo setter to now include default ResponseText strings for a bunch of new status codes, including 308.