First, I'm not a pro. I'd only consider doing this if the tub was cast iron, then I'd still have to ponder it. Most glass block is thicker than the lip of many tubs, at least the part that was flat. There are some nice frameless glass door assemblies out there. You can get powder coated finishes in pretty much any color you want, too. Many of the custom companies also offer a vapor deposited coating for the glass that makes the surface much harder and smoother - the water nearly all beads up and runs off on its own. It's easier to keep clean. PPG offers it on their glass at about $7/sqft extra. All of the nicer ones don't have lower tracks to catch hair, soap, etc and look grungy.