Indeed the excess is excreted as nicotinuric acid and wastes glycine. However what's curious is that in this experiment, excretory methylated metabolites increased markedly with nicotinic acid treatment as well, just as high as with nicotinamide; it was a single dose. And if you note, apparently there wased far more methyl molecules being consumed in the process than glycine. Not sure what to make out of it.
Well from what i vaguely remember, nicotinic acid is converted into nicotinamide eventually, which needs to be methylated. So to my basic understanding, it makes sense that both nicotinic acid and nicotinamide would consume the same amount of methyl groups.