Has anyone created a journal using a map reduce script before? Or should this only be created using another script type such as a userevent script? The use case is that the amounts for the journal are based on the results of a saved search. The balancing amount needs to be calculated as the difference between the debits and credits returned from the saved search

journal entries can be created from a map/reduce scripts

If you are creating a single journal, maybe a scheduled script would fit better. Run the search in the script, do your logic, create the je.