She finally turns up in anime episode 8.8/323, “Master of Kouan,” and the series’ Twitter teased that they got someone interesting to voice the lady.
That would be Kouka, who’s also on the cover of the Hideaki Sorachi manga’s 64th volume.
If you’ve been watching Gintama’s Battle on Rakuyo arc, you’ll have noticed that there’s a new mystery woman in the series’ opening (not that it’s THAT ambigous who she is).