I really like the Apply Image filter in combination with the equations. As I understand, the equation in Apply Image is a way of specifying the blend calculation. My suggestion would be that you can specify a name and the according equation and have them available in the blend mode dropdown list of the layers panel. The major 2 disadvantages of the Apply Image filter, is 1. It is destructive and 2. You will need to retype the equation every time (repeatability).
Using macros, you can probably work around disadvantage nr 2, but disadvantage 1 has no real workarounds imho.
The custom blend modes (when applied in a document) will probably need to be stored in the current document too, so you can still exchange images with others who don't have this custom blend mode (or you have a new install and lost your custom blend modes.
What do you think? Maybe there is a way and I don't know? Having custom blend modes would be awesome and imho it will open up a lot of creative possibilities.
