You can use good old-fashioned friends:

Live On Coliru

template <typename T>
class A {
  public:
    A(const T &id) : m_id(id) {}
  private:
    template <typename Ar, typename U> friend void boost::serialization::serialize(Ar&,A<U>&,const unsigned);
    T m_id;
};

namespace boost {
namespace serialization {
    template <class Archive, typename T>
    void serialize(Archive &ar, A<T> &a, const unsigned int)
    {
        ar & BOOST_SERIALIZATION_NVP(a.m_id);
    }
}
}

You can use the getRef() approach. This

• requires no friends (less intrusive)
• requires make_nvp (because you can’t use a.getRef() as an XML element name

Sadly, having the reference getter break encapsulation in a horrific way. I’d personally prefer to have m_id public in the first place, instead.

Live On Coliru

template <typename T>
class A {
public:
    A(const T &id) : m_id(id) {}

    T& getRef()             { return m_id; } 
    T const& getRef() const { return m_id; } 
private:
    T m_id;
};

namespace boost {
namespace serialization {
    template <class Archive, typename T>
    void serialize(Archive &ar, A<T> &a, const unsigned int)
    {
        ar & boost::serialization::make_nvp("m_id", a.getRef());
    }
}
}

Bonus points:

You can use a ‘pimpl’ style struct. You can forward declare a struct inside A<>:

template <typename T>
class A {
public:
    struct access;

    A(const T &id) : m_id(id) {}
private:
    T m_id;
};

That’s less intrusive than the getRef() approach which simply breaks encapsulation all the way. Now, you can hide the private access inside this class:

namespace boost {
namespace serialization {
    template <class Archive, typename T>
    void serialize(Archive &ar, A<T> &a, const unsigned int version)
    {
        A<T>::access::serialize(ar, a, version);
    }
}
}

Of course you still need to implement it, but this can be done in a separate header and doesn’t influence class A<> (or any of its specializations) at all:

template <typename T>
struct A<T>::access {
    template <class Archive>
    static void serialize(Archive &ar, A<T> &a, const unsigned int) {
        ar & BOOST_SERIALIZATION_NVP(a.m_id);
    }
};

See it Live On Coliru as well