The OccurrenceConstraint.Assert() method is internal - so people who write their own extensions cannot use this very handy constraint (and therefore create their own - creating inconsistency of how to deal with assertion occurences).

Understand why this is internal - so the actual usage hides this when defining an assertion for the test.
However why not add a new overload in the Execute assertion fluent API to accept a OccurrenceConstraint. i.e. as an alternative to ForCondition(...):

Execute.Assertion
    .BecauseOf(because, becauseArgs)
    .ForConstraint(constraint)
    .FailWith("Expected {0}{reason} but found {1}",
              constraint, Subject.Count());

Note there would be a formatter for the occurrence constraint to render the expected amount / "mode" (since these are also internal - and should continue to be)