MassMessage allows to create a custom footer by editing MediaWiki:Massmessage-message-footer
However, if you use wikitext syntax in the footer it will not appear formatted when previewing a message.
PS: I haven't tested whether the wikitext will be respected and formatted when the message is delivered. Since the preview looked wrong and I was in a hurry, what I did was to delete Massmessage-message-footer.
Version: REL1_22-branch
Severity: normal