This has been discussed before, and it was decided that effects that hide cards in non-hidden games would not be implemented. Several players do not play hidden mode at all, and such an effect would force them into it.
Someone had also suggested a potentially interesting mechanic where a handful (as in, no more than four total) of cards would have a keyword that renders them hidden and the opponent would have to wonder at which of the few it was unless otherwise revealed. This, too, was was ultimately decided against, or perhaps conversation on it simply never continued past that.