It's canon in my head

Ever After#1

I think Jacqueline hid the dress.

Obviously the servants knew where the dress was, so you could argue that they hid it. However, they're also a bit fearful of the stepmother and wouldn't want Danielle to get into trouble, hence why they go to lengths both great and small to hide Danielle's actions from her. (Like the coins secretly bequeathed to her, not telling the stepmother that she was out with the prince, etc).

Jacqueline stays a pretty quiet and passive background character for most of the movie, but she does give Danielle little helpful hints when she sees her mother being mean or sneaky, like hinting about Henry on the horse and trying to call her out on the lie on airing out Danielle's dowry dress for the ball. By this point in the story, Danielle and Jacqueline have started to bond and Jacqueline has almost no good relationship with Marguerite if she has any at all. Her mother just whipped her stepsister, she burned Danielle's book which was the only thing of her father's she had left, insulted her biological mother whom Danielle never met, and then continued to confiscate the few pretty and personal items she owned. Although Jacqueline still wants to keep the peace, she's starting to side more with Danielle.

So why do I think Jacqueline hid the dress?

For the simple reason that she told her mother, "It's only a ball."

When Baroness Du Ghent accuses Danielle of hiding the dress, Danielle is physically and emotionally exhausted and weak, and has just about had it with everyone's nonsense. She's getting her bite back, but it's not back all the way until this very scene where she tells her stepmother she's made it her vs them. When she's first arguing, she sounds confused and upset, though her later comment about never wanting that dress to be on a "spoiled selfish cow" might also indicate that she knows it's being deliberately hidden. But to my brain, it sounds like Danielle is as clueless as the Baroness about where the dress is. The servants obey the Baroness because she has the power to dismiss them or sell them.

So the only logical person in this moment who could have hidden it was Jaqueline.

My head canon is that after seeing the whole fiasco with the shoes and the book and Danielle being whipped, Jacqueline decided to teach Marguerite a lesson. At this point she's most likely guessed that Danielle and the prince have had romantic rendezvous (which she confirms with the servants who see that she's nicer than the rest of the household members), and tells Danielle that Marguerite would not have Danielle's precious dress. She'd handle it. It sets Danielle's mind at ease and later that day, after Danielle picks herself up and manages to meet with the prince in spite of her injuries, Danielle takes her late stepmother's wedding dress and hides it, sternly instructing the servants to not let the Baroness even know where it was. She'd eventually give up and get Marguerite a new dress. Then the servants would give the other dress to Danielle so she could go to the ball and talk to the prince.

Then Danielle gets locked in the kitchens and the Baroness takes the key.

Jacqueline tries to talk her mother out of it but gets nothing but an insult back, and the servants are left to free Danielle and get her to the ball in the hidden dress before it's too late.

Thus concludes my headcanon.