In theory, would removing an IR filter on a crop sensor camera and placing a duo narrowband filter over the sensor be equivalent to a Visible + Ha mod from Spencers/Life Pixel? If not, what exactly are they doing to modify their cameras?

Not quite. The H-Alpha mods from Spencies/LP/Kolari remove the UV/IR cut and replace it with either a clear filter of the same thickness or a UV/IR cut that allows more light on the h-a wavelength. This increases the sensitivity of h-a about 4x on average.

Replacing the UV/IR cut with a duo-nb filter will effectively force that camera to ONLY shoot at those wavelengths, which is not going to be ideal as you will not be able to shoot RGB as a full color image. You want to be able to have the freedom to shoot narrowband, but you don’t want to ONLY shoot narrowband, so you would still be better off getting the h-a mod from one of those companies and using the separate drop in filter.