I've looked into these as well. It seems to me that you may be able to use a refillable unit, place it inline where the DI cartridge goes and achieve a targeted result, and you could add the media of your choice.
But, would these produce a stable and consistent infusion of minerals or would it require the water to be used periodically to allow the water to saturate, as in a drinking system?
I'm not sure about this piece, but some I've seen contain a questionable quality of the mineral contents depending on where they are manufactured.
For now, I just remineralize with waste water.