Serialization type of internal data structures

I am in the process of making a bit vector class serializable, but
this class does not directly correspond to a value at script land,
rather, Bloom filters will use this data structure internally. Making
a class serializable involves adding a new unique identifier in
SerialTypes.h. However, I am unsure which macro to use for the bit
vector class, which serves as internal building block not exposed to
script land. (That is, SERIAL_VAL would not make sense.) Would
SERIAL_CONST2 coupled with SERIAL_IS fit here?