The main use I've seen for "upstream" vs. "downstream" is in reference to dependencies between projects - often one or more open source "upstream" projects feed

The main use I've seen for "upstream" vs. "downstream" is in reference to dependencies between projects - often one or more open source "upstream" projects feeding into a combined/enhanced for-pay "downstream" product. For that reason alone, I think using the same terms in a messaging context is a bad idea.