I think I hid Ultron as an unplayable boss on Marvel VS SNK Brawl of Super Heroes. Here's another one for you, my brother, so, if you have The Kingpin as the boss on the Spider-Man fighting game, Marvel fighting game, Marvel VS Capcom or etc. It's for example. If you want to lock your Mugen characters as the boss. Watch this:
Step 1. Double click on data folder, and then double click select.def.
Step 2. Type ;row 5 hidden bonus/boss, kingpin (as an unplayable boss in your game, it's for example), click Save. And then go to Exit. And that's it! :-) Play your Mugen game and check it out and see what happens.
If you have any problems, you'll let me know.