That is SO freaking awesome '99!
justin3009 and Vehek would best be able to advise you on the proper methodology of insertion. For now, I'd have to ask the following questions:
1. Will there be an animated background scrolling behind King Zeal?
2. If so, will the animated background be a Layer 3 effect (e.g., the mist in 2300AD buildings in CT) or a Layer 1 background?
3. Most importantly, do you need to have the party confront King Zeal in human form before he turns into this badass demon guy? If so, it would be best to make King Zeal a Size 5 sprite as opposed to making him incorporated into the background.
However, there may be tricks for getting around #3 that I don't know about. Maybe via the same coding that allows a Guardian door to change appearance on command.