eazyBI eazyBI can let you define and import change history for numerical and single-value custom fields.
A multiple-value user picker field, unfortunately, does not fall in the above categories.
Since eazyBI does not import the change history for the field, it will not be possible to report on the last assignment with the standard setup.
It might still be possible to create a JavaScript-calculated custom field measure with multiple dimensions that finds the last assignment date for the currently present users.
That might first retrieve the list of current users and then iterate through the issue changelog to find the last assignment timestamp, after which the value has not been removed from the multi-value list.
The exact script has not been created yet.