At the moment, we cannot sensibly format data that gets passed into MongoDB datasources, as Kettle expects strings only.
Kettle only accepts single strings, and the meaning or format of the string is undefined and depends on the step/context in which the string is used. There will never be a one-size-fits-all solution until we change Kettle, break compatibility for Kettle users and cause unspoken pain.
Formatting is needed for both array/list parameter and dates.
On the Kettle datasource's parameter table, change the value column into a column called "formula" (I am open for suggestions on the name, btw), which is a formula, provided by the user, to format the parameter value into a string of the proper format.
To see how that could work, look at the DrillDown parameter table. It accepts either a plain column, or you can write a formula to compute a value.
Then you finally produce the data where you need it, without having to think about hidden post-processing parameters. (And my solution even works when used as source for subreports, or parameter-queries!)