> sum is pretty clearly the more useful of the two ops and makes more intuitive sense as a default

Agreed. Sum will be the default

> would require people upgrading prototxts

Most importantly the model prototxt will break, so users will know to revise their definitions. This is too niche a layer and use case at the moment to warrant any fancy machinery. It's a nice thought all the same.

> specify coefficients for the sum inputs (a la cuda-convnet)?

Yeah, I actually had that schemed next for averaging predictions in simple ensembles and forming pyramids. I'll include it here.