What I really need is the ability to obtain the result of a pipe command so that I can filter externally and redirect internally. Is this possible?

The result of a pipe command is not available to a filter, because Exim does not run any actual deliveries while filtering. It just sets up deliveries at this time. They all actually happen later. If you want to run pipes and examine their results, you need to set up a single delivery to a delivery agent such as procmail which provides this kind of facility. An possible alternative is to use the ${run expansion item to run an external command while filtering. In this case, you can make use of some of the results of the command.