User blog comment:Xilinoc/Ch. 567 (Rukia be like "I ain't even scared though")/@comment-5147109-20140207231148/@comment-10554972-20140208025938

Basically, her Shikai is endothermic, absorbing heat energy. So it could theoretically work to cool down his heat armour long enough for an attack to get in before he regains it. Rukia would probably get damaged a little from absorbing that much energy, but I think it could work.