Therefore, you cannot do this unless you make a copy of the default model into an add-on and rename it, make the TSShapeConstructor datablock, THEN the player datablock.
Well actually, you could create a new TSShapeConstructor, inherit from the old one, with your new sequences and it should work fine.
Or you could just be referencing the m.dts directly (ie with a full path, base/shapes/whatever it was) along with all its sequences this way and just add the new sequences into it.
Actually replacing the original TSShapeConstructor however is probably not possible. I dunno if it would let you delete and recreate it. Or when it's loaded.