I recall yanfly did something in rmvx where you could put notetags on equipment that would amplify, say, fire damage by a percentage, but I've found no such thing for rmvxa.

Anyone heard of any such thing for vxa, or am I out of luck?

In the Database, go to the weapon you want to make elemental, and double click under the 'Features' box to add a new feature.
On the first tab 'Rate', you can change the 'Element Rate' for different elements to change their damage multiplier.

Is that what you are after? Or is that different?

Element rate affects how much damage you take, rather than how much damage you deal.
So if your element rate goes up, you take more damage.

Ace has absolutely no real support for elemental properties. It's just more like "lol this is a fire sword"