Actually I don't have a direct solution.
I am using a keyboard macro to copy the heading content from another org file.
In a similar link
[[file:file.org::*Heading][copy external content]]
I saved a keyboard macro,
F3 ;; to start defining the keyboard macro
C-c C-o ;; to open the file and point to Heading
C-c C-x M-w ;; to copy the subtree
C-x 0 ;; to close the buffer and switch to the previous one
C-e ;; end of line
C-y ;; yank
F4 ;; end the macro definition
C-x C-k n ;; define a name for kb macro (session only)
In your .emacs configuration you can save the macro:
Hope that can help!