Yes, there is a hidden Developers' Room, and it's in the hardtype version of FFIV, so here's how you get to it!

Go to the Dwarven Castle in the underworld. Get to the room where you can buy weapons and armor; there is a path in between both those counters. Walk up into the Rally-ho Pub. There, walk directly to the right (around the table of course) and you'll see an odd blackish section of floor. Walk right through the wall at that point and continue right and down the steps into the hidden Developers' Room!

There is also a hidden porn magazine in this room as well. Get into the Nap Room (from the entrance to the room, go up and right, then down into the Nap Room). Check out the left side of the bookcase nearby to find Smut, which is the magazine. Select it on the Item menu and the screen will turn purplish as Cecil looks at it... Then after it's over, the screen's back to normal and the Smut is gone.