Once it works dbus is easy, and here's the code to make it work. I guess you should perhaps also include glib.h, but it worked just fine without. You should be able to figure out the other functions yourself, so have fun with this piece of code!

#include <dbus/dbus-glib.h>
#include <dbus/dbus-glib-bindings.h>

static void setAWNIcon(char *app, char *filename) {
DBusGConnection *connection;
DBusGProxy *proxy;
GError *error;

g_type_init();

error = NULL;
connection = dbus_g_bus_get(DBUS_BUS_SESSION, &error);

if (connection != NULL)
{
proxy = dbus_g_proxy_new_for_name(
connection,
"com.google.code.Awn",
"/com/google/code/Awn",
"com.google.code.Awn");
error = NULL;
dbus_g_proxy_call(proxy, "SetTaskIconByName", &error, G_TYPE_STRING,
app, G_TYPE_STRING, filename, G_TYPE_INVALID);
}
}

static void restoreAWNIcon(char *app) {
DBusGConnection *connection;
DBusGProxy *proxy;
GError *error;

g_type_init();

error = NULL;
connection = dbus_g_bus_get(DBUS_BUS_SESSION, &error);

if (connection != NULL)
{
proxy = dbus_g_proxy_new_for_name(
connection,
"com.google.code.Awn",
"/com/google/code/Awn",
"com.google.code.Awn");
error = NULL;
dbus_g_proxy_call(proxy, "UnsetTaskIconByName", &error,
G_TYPE_STRING, app, G_TYPE_INVALID);
}
}