mdavey: thank you (again) very much for your support.
Both DTR & RTS are present on the "rear RS-232" and works quite well: the only thing I had to do was to patch the kernel in order to:
1) Disable the SIR setup (this was causing your problem of sudden level change during kernel operation)
2) Modify a bit get_serial_info & set_serial_info in order to enable a minimum set of ioctl needed to work with DTR/RTS from my user-mode daemon
Today, 0:40 my dashboard saw for the very first time a cd-changer that was - actually - the empeg.
The fun part of the whole project is that since 10 minutes, thanks to an enhanced interface, I don't need any RS232 output (apart the TX signal of course) anymore :-O
Let me some days to sort out various things and to refine various patches and I'll release everything on a site as promised.