I think that Quilodran's QuilodrĂ¡n's solution to the joints problem in $\mathbb{R}^n$ can be applied to the finite field case, to get the same bounds. This is the paper, "The joints problem in $\mathbb{R}^n$": Abstract:
http://arxiv.org/pdf/0906.0555.pdfarXiv:0906.0555v3; PDF link.

