> It looks from your snippet like the linker does output symbols for the beginning and end of each segment.

No only beginning and length of each segment.

> So, the next question is: can you create your own segments

You can. Although currently not without falling back to inline assembly. Feature request #1600296 which was filed very shortly before your post is related.

-dummy.c-------8<--------------------

void dummy_eo_xdata(void) __naked
{
  __asm
        .area XSEG_DUMMY    (XDATA)
        _eo_xdata::        ; nothing alloc'd!
  __endasm;
}

-main.c-------8<----------------------

extern unsigned char xdata eo_xdata; /* or void or an array[0] */

unsigned char xdata another_var[0x234];

void main (void)
{
   volatile unsigned int t = (unsigned int)&eo_xdata;
}

then on the command line:

sdcc -c dummy.c
sdcc -c main.c
sdcc main.rel dummy.rel

gives (file main.rst) :

                         128 ;       main.c:6: volatile unsigned int t = (unsigned int)&eo_xdata;
                         129 ;       genCast
0062 75 08 34            130         mov     _main_t_1_1,#_eo_xdata
0065 75 09 02            131         mov     (_main_t_1_1 + 1),#(_eo_xdata >> 8)