As far as I know, there is no way to do it outside of equipping equipment/using abilities to do so. I wanted to do that originally with the Oil status (taken from FFT, makes target weak to Fire regardless of they absorb Fire naturally or not), but it wasn't possible because of no way to reduce/increases elemental resistance with status (and looking at it recently, the only real way I see of this being done with this is through the method I'm thinking of). So unless the Class tab can be used for this purpose, there isn't many other ways to do it.
20th power? Nah, it'd only be for specific things I think (granted, when one has about 50 characters it might take a while...luckily half of the cast's equipment is locked anyways for me so that's half the work...). Then again, might be a bit confusing considering if I make various pieces of equipment interact with one another...like a Flame Shield (absorb Fire, weak to Ice) and a Ice Armor (absorb Ice, weak to Fire). I don't think I need to worry too much about statuses with this method at least...
Hopefully that clears it up some for you? ^^;;
EDIT - To simplify this:
1) Hero A has normal resistance to all elements. Hero A equips Ice Shield, giving him absorption to Ice and weakness to Fire. Since you cannot really go from 100% to -100% unless you set the element to that from one letter to another (which isn't a good idea), and since you cannot set elemental weaknesses anywhere else, perhaps the Class tab can be used here. So Class #1 would be the exact same stats as without the class, except with Ice absorption and Fire weakness.
2) Hero A also equips a Water Ring, which increases Water resistance by one level. Easy to do in accessories, so no changes needed here.
3) Hero A instead decides to equip an Aqua Ring to absorb Water. Hero A would then change class to something that absorbs Water and Ice, while being weak to Fire.
4) Hero A unequips all but Aqua Ring. Change class to match the Water absorption.
5) Hero A unequips everything and has nothing to change elements. Remove class to None.
Basically, that's what I'm seeing this being able to be used for. Can even extend to statuses that make a character weak to elements and such. This of course is based on the character having no elemental affinities (which may or may not be the case depending on the game. Considering my game has everyone with an element sans a couple characters, this is not the case here). Hopefully that cuts it down more ^^;