I got to Beijing South a few minutes earlier than usual this morning, so I did some recon on dining options.
This not comprehensive because I didn't walk the entire station, but here's a summary of my findings:
-three KFCs
-a Ramen place (downstairs)
-SPR Coffee (downstairs); I dislike coffee, but my friend said it wasn't very good
-UBC Coffee (inside "secure"<ha ha!> area); overpriced
-McCafe (ditto); I didn't go inside, but it appeared to be much larger than the typical McCafe
-BK
We ended up going with KFC; don't be shy of ordering a lot of stuff because 5 hours without food on the train is a long time.