Brien,

if you are within two months of harvest it's probably not a good idea. I believe that is the cut off line. lots of water though.

Garlic is a very heavy nitrogen feeder and agronomists claim that most all soils are deficient in sulpher.I do my last feeding about 2 months before harvest.Water soluble is best at this time including Alaska fish but not insoluble organics as they need soil microbes for their decomposition which by necessity requires being mixed into the soil,thus damaging tender feeder roots,and a longer time frame for decomposition.
Yes,ammonium sulphate will work if you have enuff time before harvest.Be sure to water it in well.