Schleuder itself doesn't have this feature yet, but schleuder-web could push for it by providing the workflow already.
Requesting subscription should trigger the sending of a token, light for the account-creation. Only this time Schleuder should send the token, and it should be able to receive and validate the response.
But Schleuder should also be able to include a link to click on, if the subscription is requested through schleuder-web, and schleuder-web should also be able to receive the response-token and validate it against Schleuder.
If we build this feature, other features could benefit from it too, e.g. the validation-emails on account-creation could be sent through Schleuder and be signed by the actual list, too.