I have published a detailed answer here: forums dot gentoo dot org slash viewtopic-p-8132756 dot html
(i subscribed only to answer this post; and recent members are not allowed to post links)

Here is the short version:

echo "#include <fcntl.h>
#include <sys/ioctl.h>
{ int fd; fd = open(\"${MySerialPort}\",O_RDWR | O_NOCTTY );
int RTS_flag; RTS_flag = TIOCM_RTS;
sleep (${MyLatency});
close(fd); } " | tcc -run -
Note that sending data on TX will probably mess RTS; see the Gentoo forum for details.

I am not sure it's possible to activate DTR and RTS pins independently from shell, it's probably possible from C background daemon; but unsure via pure shell.

This answer comes a bit late, but after several days of Googling, all similar questions are left without answer. Now, this topic has an answer for people who will have a similar issue in the future.