Dunno about the music, but Magic Resistance is mostly possible at this point. Since all spells use calculations for their damage (and some effects), it wouldn't be that difficult to throw in another calculation that reduced/increased it based on some custom UnitStats. Percentage Resistances (with a default of 1, meaning they take 100% of the normal effect) would likely work the best here.

The biggest problem here is that you'll have to set the Magic Resistance of everything you want to have to a different Magic Resistance than the default, and units are spread across a heckuva lot of files...

The files you'll want to look at are CoreUnitStats, and any of the Core spell and unit xml files.

Take a look at this thread about the Calculation tag for more info on it:

https://forums.elementalgame.com/394151

In terms of the properties of UnitStats, Hidden=0 should make it visible on equipment, at the least, and on the book entries of units you don't own, but not the ones you do, for whatever reason.

Upgradeable=1 will make it visible on the Book Entry page along with the other Champion stats, but it will also show up on the Level Up screen and might end up shunting off one of the Core stats off that screen once the Champion receives Essence, since it doesn't have scroll bars. New stats do seem to always appear on the bottom of the list, however, and ends up getting replaced by Essence once the Champion is imbued, however.