So it doesn't really constrain the picker? If I have a client in one OU and another in a second OU and do not want them seeing or guessing each others users or groups, that would not be possible. I just need to type in the name of the group and resolve it.
EDIT: Maybe I'm working in a different kind of picker but this behavior isn't happening for me.
I also have the peoplepicker-searcadcustomfilter with a query. Is this completely ignored when search in the site collection only is set? It seems to be.