an answer with respect to the DSC targets (and other related topics) you will find in your on-line manual (the "?" icon) under
advanced features | AIS
You will find as well extended documentation
about marks and waypoints in the manual.
Creating a mark, waypoint or POI from received $WPL sentences would be a new feature - perhaps those might be placed automatically into a separate layer. Just an idea.
A feature request can be created in Flyspray, the bug tracker for OpenCPN:
OpenCPN::Tracker All Projects: Tasklist
Registering is required - just takes a minute.
btw.: Dirk was faster than me...