My company has a vendor coming in this afternoon to do the dog-and-pony of their workflow product. It occurs to me that a workflow engine is just really a rules engine with a bunch of rules already implemented, right? Would that be an accurate assessment, and if yes, do you know of workflow products based on JESS that allow extension by custom JESS rules?
Your assessment is more or less correct, yes. The rule engine in a workflow system only needs to do a very limited number of things in response to rules firing -- create new workflows, update workflow state, and send email, more or less -- and the inputs are similarly limited, so they can often get by using only a very rudimentary rule engine. I don't know of a commercial workflow engine that uses Jess, but then, I'm not always aware of all the things licensees are doing. We've had some discussions of Jess and workflow engines here on JavaRanch recently; try searching in Java in General (Advanced.)