ternary is totally losslessly compressed anyway.

Why not just use an 8-bit LUT to encode the 256 most common ternary vectors with 6 components. That means of the possible 729 possible such vectors, you can only represent 256 different ones. You have to do more aggressive rounding, but at least the scheme is very simple to decompress and stream.

I think the weights are iid distributed, so all 729 patterns will be roughly equally likely. That doesn't make this a bad idea though -- it just means there's no point trying to select the most common 256 to keep, since any 256 will be roughly as good.