I have no clue, but the existence of the kaikaiups gives the option and knowing how Sengoku Rance takes a few months to clear blind without pulling allnighters I would say it's not beyond Alicesoft to plan that far ahead. King Hanny was a bit too obvious to be a secret boss anyways.
Feel free to try it yourself and tell us the results, I'm currently busy backtracking into NG to clear the already opened doors-unfortunately I entered super-grind mode after I entered the Revolution door so I'll have to do everything again (including the fragging bosses) and I'll probably delete my NG+ file.
Wonderful I found something even WORSE than King Hanny. Valentine regenerates 800+hp every turn, has double turns, hits like a truck and causes your allies to attack eachother. It's THE most aggravating thing in the world to get her down to low hp after around thirty turns, then have Rance start killing everyone(better yet they counterattack him and kill him), all while Valentine regenerates all her hp back in the chaos. Beat Valentine-Tillday is the ultimate boss killer, try to get as many of her star upgrades as possible because her specialization in proc'ng double action basically gives her X2 mobility, damage, chance to crit and upgrades her special attack-on this boss fight her special proc'd five times with overcharge to deal around 900dmg.