Use assuan_register_pre_cmd_notify(). This is a patch to libassuan.
tree36c8e985d50e7596aa71397a305a8068552ab871
-rw-r--r-- 17989 COPYING
-rw-r--r-- 9416 INSTALL
-rw-r--r-- 0 KnownBugs
-rw-r--r-- 347 Makefile.am
-rw-r--r-- 22691 NEWS
-rw-r--r-- 4038 README
-rw-r--r-- 1759 TODO
drwxr-xr-x - assuan
-rwxr-xr-x 2412 autogen.sh
-rw-r--r-- 6124 configure.ac
drwxr-xr-x - debian
drwxr-xr-x - doc
drwxr-xr-x - po
drwxr-xr-x - src