This one had me completely mystified, and found no one who had even the slightest clue, until a friend of mine in Paris finally tracked down an attorney friend of hers who sent me the following explanation, which is actually rather amusing. "Toque," of course, is, in its primary definition, a type of hat, but by extension, has become the attorney's box at the courthouse:
un mot désuet que correspondait au numéro attribué aux avocats autrefois au vestiaire du palais pour ranger leur... toques et leur remettre leur courrier émanant des juridictions siègeant au palais de Justice - dorénavant. Toque se dit aussi "numéro de vestiaire" et correspond donc au numéro de case de l'avocat au palais de justice.