I have identified the causal mutation in sna18. I got a sna18 stock from Bloomington, rebalanced over a GFP CyO balancer, hand-sorted homozygous mutant embryos by fluorescence, made RNA, did RT-PCR of the sna coding region (from two independent sets of hand-sorted mutant embryos), and Sanger sequenced.
In addition to several silent (synonymous) SNPs in the coding region, there is a G->A transition at  chr2L:15 ,477,138 (dm6). This causes an H324Y mutation in the Sna protein sequence, and H324 is one of the Zn-coordinating histidine residues in one of the DNA-binding C2H2 Zn fingers.