Disclaimers are use to sign text/html and text/plain messages, regardeless of their character encoding. So only the ASCII subset of characters will be displayed correctly with all mails.
Anyways, we should notify the admin instead of displaying strange characters - i filed a bug for that issue.
So what is the suggestion? We can try to automatically convert the charcter set for the disclaimer, but what if conversion is not possible? Simply do not add the disclaimer in such cases (such cases should be rare anyways)?
When conversion is not possible the disclaimer should be added without conversion to the email. Better a disclaimer with wrong characters as no disclaimer.