After you unlock the maid outfit, you can put it on any time by going back to Karen's closet. Currently however, you can only walk around Missy's house, Emily's house and Lady Flower in it, she'll refuse to go anywhere else.
I post a weekly public update to my Patreon (www.patreon.com/trinian) and Subscribestar (www.subscribestar.adult/trinian) that already have some details of what's coming in 0.4 up on them, so give those a look ^^ The next public post will be up later today. No firm release date yet.