I've not needed to do this on DataPump but using batch files with Monarch I'd try this approach.
Assume that you have a field "ReportDate" which contains the 09/10/2009 value. Also assume that the path to the file created by your input distribution is defined as a calculated field (again, this can be hidden) called "InputPath". The original filename can be created as a third hidden calculated field called "OriginalName" defined as rsplit(File();2;"\";1). "NewName" can be created as you would like it to appear - perhaps using an expression like:
Nake a calculated field in your Monarch model (it can be hidden so it won't appear in the table) called "BatchCommand", as a long character field. If you're using very long network paths and filenames this approach may hit snags...
BatchCommand should be defined as:
Then create a summary in your Monarch project which has this field as the only key, and Count() as the only measure. Hide the measure. Export the summary as a fixed width text file called "RenameCommand.bat" to an appropriate location. Run a post-process script in DataPump to call "RenameCommand.bat".