As you say probably initially it could work by nodes observing which chunks are requested frequently as they pass via routing. Although to my knowledge routing messages only happen with elders and do not touch adults (see RMD “The Delivery Group is a subset of the Elders of the section”), so that would also have implications for cache hits/misses as well as opportunities to fill the cache.
But I reckon a system that allows sequentially named chunks to be requested as suggested here would probably be a good way to warm caches and maybe also allow an aspect of audit and accountability.