CC, There is a great waterproofing product simply called Professional Water Seal, available from ICI Glidden, that MIGHT be just the thing for you.
It is a liquified silicone which can be applied with a pump-up garden sprayer. If the tile is not sealed right now, the product is absorbed into the tile & grout, and when dry it will not have a glossy look, in fact on concrete you cannot even tell it's there until it rains; then you can tell because the concrete does not absorb water, it beads off of it.
You can check with a ICI Glidden store about their thoughts on using it on saltillo; I'm pretty sure it would work, but I have a little reservation that it could become a slip hazard when wet. It has a very good warranty against UV breakdown (I think it was 99 yrs.), but it is a pricey item, about $75 a gallon. But...a gallon will go a looong ways with this material. Might be worth a shot; good luck.