For my parks style wig, I used a base wig and several long ponytail clips from Arda.
As you said, the parks wig is 2 parts: the base wig on the top of the head and then the braid, which is attached at the base of the neck and to the dress. I didn't want to attach the hair to my costume (since I wanted the freedom to be able to wear my short wig with it too).
What I did was style the base wig like the parks (dutch braids around the side and pinned up) and then sewed several falls together (I'm not sure how many I have in there at this point as I kept adding to it) and sewed this to the base of the neck of the wig. While I almost always wear it over my shoulder, I've worn it down my back as well with no problems.
My braid is all hair (and from what i can tell from visiting Rapunzel many times, so is hers) and it weighs quite a bit, but it's manageable with it over the shoulder. I have another fall that's clipped to the bottom of the braid for the loose part at the end.
Unless you want to take the braid off and restyle the wig for other costumes and then restyle it for Rapunzel, it's probably not the best idea to use this wig for other costumes.