The torque load on the hub is determined by;
The force on the pedals x the length of the cranks x the teeth on the cog / teeth on the chainring.

So all things being equal, more teeth on the rear = more load on the hub. However things are never equal. With the exception of cargo bikes for heavy loads, anyone looking for oversized cogs is doing so because they don't apply as much force to the pedals as someone who gets by fine with an 18t.

I don't make 28t cogs, but I do make 25s by welding a 25t Sturmey 8 speed cog (which fits an oversized 63mm driver) to an 18t cog. http://bikesmithdesign.com/SA/25t.jpg Price is $20 and how you keep them from rusting in the area of the welds is up to you.

Let me know if you do find 24s. Last I heard Harris was out of them. I have been told by SRAM-USA that they are no longer made. However the US guys aren't all that internal hub savvy, so they may still be available in Europe. Perhaps Sheldon can clarify whether 24s are or will be available.

Mark Stonich
mark@bikesmithdesign.com

BTW the best way to get low gears is with a small rear wheel. http://bikesmithdesign.com/MyBikes/20/20-2.jpg