It's been years since I touched "Get Next", but Work-.GetNextWork itself is Available and its purpose appears to be predominantly to 'wrap' the inner Final activity getNextWorkObject that does all the 'work', then to orchestrate what Pega should do next based on whether work was found to do.
If you've done the digging already and can't find an extension point from within getNextWorkObject, you could consider:
Determining how to check the result of getNextWorkObject to determine:
a) whether work was found to do (hint: check the Jump configuration for the call to getNextWorkObject); and
b) on what page it resides on the clipboard if found;
Specializing Work-.GetNextWork into your application where you need the extension point
(it may have to remain on Work- ... depending on how GetNextWork is invoked);
Implementing an (empty) extension point in your application rulesets
(again, on Work- depending on the findings from the previous step);
Adding a step to your specialized activity that calls your new extension point when work is found, using the work object page of the found work item as the step page (it looks like Step 4 onwards is the "happy path");
Class-specialize your extension point so that the flags for Case Type 1 can be configured (and tested) independently of the flags required for Case Type 2. Leave the extension point unspecialized for Case Types that need no special behaviour (Case Type 3).
As you are cloning a Platform rule, this introduces an upgrade risk should Pega decide to evolve the implementation of GetNextWork or the 'contract' of what getNextWorkObject does (where it puts its found work item, how it broadcasts that a match was found/not found). I recommend keeping a log of specialized platform rules like this so that scenarios for them are included in regression testing of upgrades.