My wife is a very active “chatzoser”, as they like to call themselves. Chatzos is a wonderful tool to use in helping yourself be goal oriented in time management.
Chatzos is not meant to change the halachos of sheshes yamim taaseh melachtecha. One may NOT be mekabel Shabbos upon oneself before plag hamincha on Friday.
The story with the Chofetz Chaim that served as the catalyst for the movement, had the Chofetz Chaim giving advice to someone looking for bracha. He told that one individual person to have his table set, and Shabbos licht prepared by Chatzos. The Chofetz Chaim NEVER forbade melacha after Chatzos, as that would be k’neged halacha.
As long as Chatzos is used as a tool to promote proper planning for Shabbos, and helping alleviate the stress so often found in Jewish homes on Fridays, then it is a wonderful, admirable thing. Please do not use it to change Shulchan Aruch.