I have it in the inventory, but it does not appear as an option at Rose's wardrobe ?! Neither do I see a dialogue option to give it to her - what's the trick to let her wear it?

just gift her it. when she is at the bar.

Doesn't work ... The options there, or at the Crystal pool at night, or in the morning at her house, are always "Talk / Mansion / Nevermind" only, limiting the outfits to Default, Feline and Rabbit  ... I prefer Rabbit anyway, but I'd still like to have the full choice ^_^

well i gave it to her in the evening at the bar. it worked for me.

Probably a bug that prevents it after doing the Halloween sidequest ... but thanks for the info anyway.