So I ended up implementing DDE Execute commands in python, using ctypes. The code is in the LaTeX package: see the file DDEExecute.py, which implements the "sendDDEExecute" command, with the following syntax:
For instance, to get SumatraPDF to jump to a point in the file "file.pdf" corresponding to line 100, column 1 of "file.tex" (actually, the column is disregarded at this time), the command is
sendDDEExecute "SUMATRA" "control" "[ForwardSearch("c:\file.pdf","c:\file.tex",100,1)]"
Maybe this may be useful in other applications (e.g. telling a web browser to reload the current page, etc.).
The file DDEExecute.py is independent of the LaTeX package; it is only used by it.