Though RegReplace does support fold, unfold, mark, unmark, I am not sure it is all supported via "on_save". A matter of fact, I am pretty sure it isn't...but I could add it. RegReplace is a plugin that I plan on spending some more time on some time soon. It was thrown together pretty quick to fulfill my basic needs, and while it works well for what I use it for, it has some gaps in features etc. and has some areas that could use better polish.
If you hop on github and create an issue for allowing for fold on save, I will try and add it sometime soon.