Well, to be perfectly honest, even a transforming valk at that scale would be manageable. There was someone on MOCPages who was making a PT Lego VF-1S about 3 feet long. Though he never accomplished the legs, the torso was working. It transformed completely and the arms were okay. He had problems, if I recall, with keeping some of the joints together, and it weighed a lot, but then, he had to make it solid to keep the design aesthetically accurate.
I think if good enough ratchets were added, joints would be no problem, and even at 3 feet long, weight shouldn't be too much of an issue. I mean, sure, yeah, it'll weigh more than any other valk, probably any other 1/18 plane, but it shouldn't exceed a feasible weight. I mean, it'd be something you could carry for a while in your hands, definitely.