Removal of lime buildup in toilet bowl?

I'm maybe missing something, but I can't understand the problem. The main drain path inside a toilet has a very large opening that should not be affected by a coating of calcium/lime from hard water. If you're experiencing poor flushing, you won't fix it there. (Clogged water ports under the bowl rim will, but that's not what you asked about.)

Try using a combination of baking soda and white vinegar (equal parts). It won't hurt the porcelain or anything. It will boil up. Let it sit for awhile, and then try removing the residue. You might have to repeat it several times.