The nice guy at Home Depot gave me a pro tip on setting marble tiles flush: put down the mortar, put down the tile, then run a thin bead of color-matched caulk along the edges and put down the next tile, snuggling it up to the first one. That way it's sealed (essential) and the sealant line is nigh-invisible. Then cut off the excess, re-caulk along the top, and while it's still wet, spray the whole floor with windex or something else slightly soapy, and strip off the excess caulk with the edge of something hard. That way the caulk won't stick to the adjacent tiles, and you'll end up with something that looks just about seamless.
Nice idea. If I were way better at tiling it might've worked. As it was, the caulk got down in the setting mortar, and when I nudged adjacent tiles up beside each other, a slurry of mixed caulk and mortar squoze out between them, making clean-up very difficult.
Neat idea, tricky implementation.