I've just spent many unproductive hours trying to work out why the hidden portal trick wouldn't work in a new FMP12 build, despite having done it many times in FMP7-11. I even built a test FMP11 file which worked fine, but exactly the same setup in FMP12 would not. I eventually discovered that you have to draw the portal first, then drop the objects to be hidden into it. If you draw the portal over existing objects on the layout, it won't work. This, presumably, is a by-product of FMP12's changed behaviour in handling objects inside portals. Now, when you drag a portal, all objects inside it move with the portal. Up to FMP11 you had to select the portal and all objects inside it. You can test if the hidden portal trick will work by dragging the portal. If the objects inside move with the portal, it will work - if they don't, it won't. Cut the objects to put them onto the Clipboard, then paste back in.
(The 'hidden portal' trick for those who don't know it allows objects to show or be hidden at will. It relies on portals not showing content if the relationship is invalid. Switching is generally done by changing the value in a global field on one side of the relationship. It's an undocumented feature that's been available for many versions.)
Hope this saves hours of future head scratching by others.