The engine source code does not underlie any license or contract restrictions, ""only"" the lite-C compiler and the editor source codes.
I once asked the support if I could get source code access, primarily to fix some bugs, but in general to make life easier for me. The price however was way, way beyond any reasonable point, esp. when you keep in mind that there won't be A9 (and that there's UE4 source code access - with royalties though).