I'm currently porting (https://github.com/zevlg/ffi-wand.el) nice wand-mode from SXEmacs to GNU Emacs  and I miss feature to specify image `:data' as user-ptr object.

There is also no possibility to create DATA string in emacs-module, without string interpretation, because there is only `make_string' method, which expects given `str' pointer to be null-terminated UTF-8 string.