OK So I found the answer, as explained here:
http://msmvps.com/blogs/laflour/archive/2008/05/20/publishing-content-types-and-missed-quot-createpage-aspx-quot-page-in-wcm-sites.aspx
All you have to do is to set the DocumentTemplate property of the content type to:
/_layouts/CreatePage.aspx
so in my case:
var pageCt = list.ContentTypes["MySitePageContentType"];
pageCt.DocumentTemplate ="/_layouts/CreatePage.aspx";
pageCt.Update();
Since I am deploying via a feature event receiver, if you doing it via an xml maniphest:
<ContentType
ID="0x010100C568DB52D9D0A14D9B2FDCC96666E9F2007948130EC3DB064584E219954237AF3900<GUID>"
Name="my Publishing Content Type"
Group="my Publishing Content Type"
Description="Content Type inherited from Publishing Page"
>
<FieldRefs>
<FieldRef ID="{<GUID>}" DisplayName="Title" Name="Title" Sealed="TRUE" />
<FieldRef ID="{<GUID>}" DisplayName="Description" Name="Comments" />
</FieldRefs>
<DocumentTemplate TargetName ="/_layouts/CreatePage.aspx"/>
</ContentType>