Maybe this is just an option that should be enabled for meta:Grants namespace? Reporting, just in case.
I wanted to sign my endorsement at https://meta.wikimedia.org/wiki/Grants:TPS/01tonythomas/FOSSASIA2017#Endorsements without having to leave the visual editor, but couldn't because the option was grayed out. I could sign with the 2017 source editor, though.
Consistency between both editors. In fact, I (personally) would expect the tool to give the choice to sign, even when the user is perhaps wrong, unless the tool is completely sure that I am not supposed to add my signature on that page.
The visual editor doesn't allow to add the signature on a page welcoming them.