The issue has been discussed on the mailing list. Please find attached a proposal for a new class UnmodifiableRealVector. I chose not to nest it in AbstractRealVector because it would make the corresponding file huge. Therefore, UnmodifiableRealVector is final. Maybe you'd like it to be private as well? A static method is provided in AbstractRealVector to build an UnmodifiableRealVector from any RealVector.
Tests are also provided. Since iterating through different implementations of RealVector is actually different, a test is provided for UnmodifiableRealVector built on ArrayRealVector and OpenMapRealVector. These tests both derive from the same abstract test class. Hope everything works fine.