At least some of them are attached; CTS is used for dimmer sense, DCD for mute.. it would seem logical to assume that RTS his other friends are connected too. I don't actually use the mute line though so hijacking it would be fine.

empegcar.h defines some useful looking constants:

#define EMPEG_SERIALDSR      (GPIO_GPIO21) /* IN  Serial DSR                 */

#define EMPEG_SERIALCTS      (GPIO_GPIO22) /* IN  Serial CTS                 */

#define EMPEG_SERIALDTR      (GPIO_GPIO23) /* OUT Serial DTS (also LED)      */

and empeg_power.c has some code that would seem to work:

	unsigned int gplr=GPLR;

...

	if (!(gplr&EMPEG_SERIALDCD)) bitset|=EMPEG_POWER_FLAG_EXTMUTE; /* Tel mute */

, but doing anything with GPLR seems to cause a segfault (which dumps to the display, and half the chars are dropped because it's not observing the CTS line ). Maybe it's only accessible in kernel space?

Software flow control would require the cooperation of the display firmware, which isn't there.