Usually addresses for variables are only known after the linking stage
you did'nt hear? 'usually' died yesterday.
should compile to something like:
and so did 'should'

I always define all XDATA in an asm module and as externs in a .h file.

two advantages;
1) you can locate fast stuff in the internal XRAM and slower stuff in the external one (in my case - and I guess many others - the extenal RAM is slower than the internal 8k)
b) if a buffer is at a boundary, you do not need to count or calculate to see byte 47 in the emulator.

I doubt any compiler maker would even consider taking adavantage of an _at_ (it is so rarely used) for a purpose like this. So, if it has to be tight (some does - but not as much as many think) asm rules.

a general comment on _at_:
_at_ is fine for locating MMIO (if you still use it) but for general variables you are likely to generate holes and overlaps (the linker - hopefully - will tell you about overlap, but I think not about 'holes').

Erik