It's pretty straightforward to send messages to objects in a c external using functions like pd_typedmess (e.g pd_typedmess((t_pd *)gensym("pd")->s_thing, gensym("dsp"), 1, argv) ).

But is there a canonical way to send object creation messages to the parent canvas: for example (msg 300 10 bang)?

I think you can use pd_typedmess but just use the specific t_canvas or t_glist as cast to the (t_pd *) instead.
use gensym("obj") to send an object to it, and then the argc/argv type atom list for the object name, x & y coordinates, arguments, etc. (it is a method with A_GIMME)
if you have the pointer to the t_canvas or t_glist struct like t_canvas* mycanvas you can get the parent with mycanvas->gl_owner
edit: you will need to include g_canvas.h for that I think

Thanks for your reply, Seb,

Despite your advice, I'm still not sure what the correct way to go about it. The following segfaults.

    t_glist *mycanvas = canvas_getcurrent();
    t_atom av[4];
    SETFLOAT(av+0, (t_float)300);
    SETFLOAT(av+1, (t_float)10);
    SETSYMBOL(av+2, gensym("r"));
    SETSYMBOL(av+3, gensym("test"));
    pd_typedmess((t_pd *)mycanvas->gl_owner, gensym("obj"), 4, av);

Any further tips would be much appreciated.

@shakfu canvas_getcurrent is only valid when the object is being created. Put it in the "new" function and store the pointer (or pointer to the parent) in the object's data structure and use that in methods instead of calling canvas_getcurrent in the methods. (if you aren't calling it in the "new" function)
also make sure gl_owner actually exists (might want to check e.g. if (mycanvas->gl_owner))

Thanks Seb! Very much appreciate the detailed explanation.

I managed to get it working with your advice.

Here's a demonstrative case:

#include "m_pd.h"

static t_class *demo_class;

typedef struct _demo {
    t_object x_obj;
    t_glist *mycanvas;
} t_demo;

void demo_bang(t_demo *x) {
    t_atom av[4];
    SETFLOAT(av + 0, (t_float)300);
    SETFLOAT(av + 1, (t_float)10);
    SETSYMBOL(av + 2, gensym("r"));
    SETSYMBOL(av + 3, gensym("test"));
    pd_typedmess((t_pd *)x->mycanvas, gensym("obj"), 4, av);
}

void *demo_new(void) {
    t_demo *x = (t_demo *)pd_new(demo_class);
    x->mycanvas = canvas_getcurrent();
    return (void *)x;
}

void demo_setup(void) {
    demo_class = class_new(gensym("demo"), (t_newmethod)demo_new, 0,
                           sizeof(t_demo), CLASS_DEFAULT, 0);
    class_addbang(demo_class, demo_bang);
}