The defense wants an instruction in a capital case telling the presiding juror not to sign the verdict form if they are not a unanimous "Yes" or at least 10 saying "No" to one or both of the special issues in the capital jury charge. Anyone know of a case discussing this?

Not precisely, but take a look at McFarland v. S., 928 S.W.2d 482, 519 and Prytash v. S., 3 S.W.3d 522, 536.