It depends on what you mean by Grand Dragon. The current Dragon model in game already supports a PPC in the right arm so a simple json edit can allow you to implement a Dragon 1G with a rendered PPC which would satisfy many as a 'Grand Dragon'.The Locust 1E and Spider 5K already exist as part of PGI models. The Grand Dragon does not. HBS *can* yes, but beyond switching out weapon PGI hardpoints to support mounts they haven't made new ones. New models are different as they are built from the ground up to implement the hardpoints. I'm not saying they can't I'm saying it's highly unlikely.
As someone who has spent too much time modding in new mech variants I'll point out that most chassis in the game have many more hard points implemented than what the variants HBS have included make use of. Off the top of my head beyond the Dragon with a PPC in it's right arm the Zeus has a hard point for a PPC in it's left arm and the Battlemaster has missile hard points in it's arms so you can easily make a 1S variant.
Beyond that there are all sorts of oddities such as the Cyclops with two autocannon mounts in each torso, the Raven and an autocannon mount in it's left arm and even the Panther has an autocannon mount in it's right arm. All this is a great boon for those like me who want to mod in all sort of weird variants but have no artistic talent.