Is there a tool to convert an edmx into code-first? I know there was talk of one appearing in a CTP a while back, but I can't find any updates relating to this.
There's a guy on the MSDN forums who has written his own (not available yet), but nothing from the EF team.
There is no such tool because EDMX offers much more features which cannot be translated into code-first. The only tool available are EF Power Tools CTP1 which allow creating code-first mapping from existing database but it will only create 1:1 image of your database = naming based on database, no inheritance, no splitting, etc. Power tools also allow creating read only EDMX from existing code-first mapping.
With EF6 Tools & Visual Studio 2013 or Visual Studio 2012, you get the option code first from database (see screenshot below)
If you don't see this option you need to install Entity Framework 6 Tools for Visual Studio 2012 & 2013 http://www.microsoft.com/en-gb/download/details.aspx?id=40762
This doesn't convert an EDMX to code first (as requested in question), but it can create code first from an existing DB (which I assume someone wanted to do this from EDMX, would have an existing DB, unless they deleted their DB).