Sorry, I don't do brick. But for another opinion see Brick & Tiles Unlimited. The fact that it is an unfinished tile is irrelevent. This is why pre-grout releases are so widely recommended for Saltillo tiles.
PS- If you are aware of a tile manufacturer that recommends muratic acid (however diluted) to remove residue of any type from their Saltillo tile line please post that info.
No matter how much the muratic acid solution is diluted (although you stated "add another cup or so") the water will evaporate and the HCL acid will remain behind as crystals trapped in the tile waiting for the next time it is subjected to cleaning with who knows what chemical. Mexican tiles are also problematic due to a high percentage of them containing pockets of lime in the clay body. HCL acid reacting with this lime (water can also cause this) can cause "lime pops" a condition where the lime expands, creating not only crater like voids in the tile surface but in extreme cases tile fractures. Even if you were avoid the facts and decided to etch your floor, wouldn't you want to etch the entire floor and not a 4' x 4' section?