*RefTeX Select* buffer you can type
C-h m to see all the keybindings in effect for the mode. From reading the keys available there doesn't appear to be a way. In fact, I don't even see a way to skip to the next heading of the same level. That seems quite useful, and probably not hard. I'll put it on my list of things to do someday. Hiding may be harder, I don't know.
You can set the maximum level with
t. For example
2 t will only show the highest 2 levels. Just
t will show all levels. This isn't specific to one chapter, but it may be useful. Another possibly useful command is
z to jump to a given section. So
3 z will jump to chapter 3. Unfortunately it only works for the top level.