The UserInputPanel field processors described in the documentation are currently broken, but even if they would work as desired in the current code they should be enhanced by the following features:
Add an optional <configuration> section nested to the <processor> definition to be able to add parameters to the processor, which can be gathered in each custom processor implementation by
in the Processor#process method.
Add an optional attribute backupVariable as part of the <processor> definition holding the original value before processing if defined. For example, a typical usecase would be holding the plain password before encrypting it in order to use it for post-installation tasks, for instance to apply SQL scripts against an authorized database.