"Chow-chow” also meant a mixture or miscellany of things in general, comparably early, I think. Therefore one can hypothesize (only hypothesize!) that “mixture” is the primary sense, specialized to food in the sense of mixed dishes such as typical Chinese mixtures with rice.
Here is an example dated 1788 from G. Books: //The _Chinese_ dressed their portion [of fish] differently, making a mixture with rice, and other things, which they call _Chow Chow_.//
Incidentally, MW3 speculates “perhaps from Chinese (Pekingese) _chiao3_ meat dumpling”. I wouldn’t bet on this myself.
One might look for a candidate etymon like “chow” in (likely southern) Chinese meaning “mixture"/"hodge-podge" or so. The reduplication (apparently in Pidgin) may or may not have been present in the Chinese original.