I notice you have added support for specifying the format of expressionPreview fields, which is great when handling dates and numbers, so thanks for that.
However, ideally I need to be able to use a template for the expression preview for certain fields. For example, take the scenario where you are filtering for 'user_id'. Currently I'm using a custom editor to present a drop-down list of user names to pass the user_id to the filter, which works well, but the expression preview looks like:
User Equals '132'
User Equals 'Fred Bloggs'
If I could use a template for the expression preview, I could use a function to display the name associated with the user_id.
This is just one example: there are many others where a template would be useful in the Filter Panel expression preview.