How can I make a (horizontal) tunnel through sand?
Solution 1:
I don't think what you're trying to do is possible, but the easiest solution in this case is to just dig your tunnel under the desert a little bit deeper. Eventually the sand gives way to stone if you keep going down, and it shouldn't even take that long to get there. Make a stairway down until you have an ample buffer of stone above you and then dig your tunnel across.
Solution 2:
It is possible in multiplayer for one person to dig the sand while another inserts a block while the sand is falling.
The sand creates a void long enough while falling for another player to quickly add a block of dirt and stop the sand from falling.
Solution 3:
I've developed a technique that's slow but better than mining the whole thing out.
- place a bomb at the base of the sand you are digging through
- jump and grapple about 4-5 blocks above it
- be ready to place a stone block in immediately at the time of detonation
It's slow work, you go through a lot of bombs, and it's very easy to blow yourself up. But hey, anything to preserve the ton of sand above it.