Ralf W. Grosse-Kunstleve provides these notes:

Below is a small, self-contained demo extension module that shows how to do this. Here is the corresponding trivial test:

import custom_string
assert custom_string.hello() == "Hello world."
assert custom_string.size("california") == 10

If you look at the code you will find:

The custom converters are registered in the global Boost.Python registry near the top of the module initialization function. Once flow control has passed through the registration code the automatic conversions from and to Python strings will work in any module imported in the same process.

#include <boost/python/module.hpp>
#include <boost/python/def.hpp>
#include <boost/python/to_python_converter.hpp>

namespace sandbox { namespace {

class custom_string
{
  public:
    custom_string() {}
    custom_string(std::string const &value) : value_(value) {}
    std::string const &value() const { return value_; }
  private:
    std::string value_;
};

struct custom_string_to_python_str
{
  static PyObject* convert(custom_string const &s)
  {
    return boost::python::incref(boost::python::object(s.value()).ptr());
  }
};

struct custom_string_from_python_str
{
  custom_string_from_python_str()
  {
    boost::python::converter::registry::push_back(
      &convertible,
      &construct,
      boost::python::type_id<custom_string>());
  }

  static void* convertible(PyObject* obj_ptr)
  {
    if (!PyString_Check(obj_ptr)) return 0;
    return obj_ptr;
  }

  static void construct(
    PyObject* obj_ptr,
    boost::python::converter::rvalue_from_python_stage1_data* data)
  {
    const char* value = PyString_AsString(obj_ptr);
    if (value == 0) boost::python::throw_error_already_set();
    void* storage = (
      (boost::python::converter::rvalue_from_python_storage<custom_string>*)
        data)->storage.bytes;
    new (storage) custom_string(value);
    data->convertible = storage;
  }
};

custom_string hello() { return custom_string("Hello world."); }

std::size_t size(custom_string const &s) { return s.value().size(); }

void init_module()
{
  using namespace boost::python;

  boost::python::to_python_converter<
    custom_string,
    custom_string_to_python_str>();

  custom_string_from_python_str();

  def("hello", hello);
  def("size", size);
}

}} // namespace sandbox::<anonymous>

BOOST_PYTHON_MODULE(custom_string)
{
  sandbox::init_module();
}