How to refer to a coll from custom external

Hi folks,

I am writing a custom UI external that manages lists of values.
It would be great to give the user the ability to use coll features to manipulate the list.

So, what I want to do is to refer to an existing coll object from inside my external.
Please note that I don't mean creating a NOBOX coll object inside my external.
What I mean is that if there is already a coll instantiated in the patch I want to be able to connect to it.

Ideally, I would like to create a state where my external and the coll are sync'd to each other, so that if you change the content of one the changes are reflected in the other. Similar to what jit.cellblock does with - precisely - the 'refer' message.

I know how to send remote messages from my object to the coll, so that I can update its state with the state of my object. I also know how to attach my object to the coll, so that I can receive coll's notifications. However that's where I am having problems...

In code:

void myobj_refer(t_myobj *x, t_symbol *sym)
{
    t_object *thing = sym->s_thing;

    if (thing == NULL)
        return;
    if (NOGOOD(thing))
        return;
    if (!object_classname_compare(thing, gensym("xcoll")))
        return;

    object_attach_byptr_register((t_object *)x, thing, CLASS_BOX);
    x->xcoll = thing;
}

t_max_err myobj_notify(t_myobj *x, t_symbol *sym, t_symbol *msg, void *sender, void *data)
{
    if (x->xcoll == sender) {
        if (msg == _sym_free) {
            object_detach_byptr((t_object *)x, x->xcoll);
            x->xcoll = NULL;
        }

        object_post((t_object *)x, "msg: %s", msg->s_name);
    }
    return jbox_notify((t_jbox *)x, sym, msg, sender, data);
}

Now xcoll is a pointer to the coll object instantiated in the patch and I am able to receive notifications from it. Great!
Problem is that the only notification I receive from coll is a _sym_free message when the coll is deleted from the patch. Any other operation performed directly on the coll (such as inserting or sorting) I am not able to detect. Also, I am not receiving any notifications if I change the 'embed' attribute, which is the only attribute coll has. Normally, I should receive a _sym_attr_modified message.
Last but not least, any message that I send remotely generates an error in the max window such as:
xcoll: doesn't understand "insert"

Please, someone help... I am completely lost...

Thank you so much.

- Luigi