It does seem like it’s a pretty well-known pattern already, and makes a lot of sense from a ‘space-saving on small screens’ perspective. I think ‘login’ would also be a reasonably natural fall back to press even if you weren’t initially aware there might be a ‘sign up’ option in there.
Let’s close this one off. 
(Though it’s worth also noting that if a particular site did want to introduce this then it would be possible using a custom theme component)