Personal email signature unusable for mailings

Personal email signatures were introduced in 3.1 CRM-5831

If you define a email signature for your own CiviCRM contact record and then try to send a CiviMail to another contact, then your email signature will be automatically included in the email when drafted. No problem.

However, if you select a Mail Template for the email then your personal signature is automatically removed - replaced by the Mail Template contents.

There is no way (I can see) to re-include your personal signature in the email. And since there is no token defined for your personal signature, there is no way to update the Mail Template.

The only token for signatures available is the Contact Signature which is not useful at all, since the signature of the current user is required, not that of the contact being emailed.

A token defined as the "Sender Signature" or similar is really required.

This bug effectively prevents personal email signatures from being used at all, the feature introduced in CRM-5831 is not working.