I agree , adding nitrogen after the 50% mark is not a good idea.
even if you smell some h2s . this is because at this late stage of life the yeast cant metabolise it , so the nitrogen will remain in the wine unused , lurking in the back ground for spoilage organisms like brett to make use of.
you didn't mention any h2s smells so its likely your must had enough naturally present yan combined with the fermaid you did add to carry it through to dryness safely.
if you do get h2s and its past the 50% mark a product like noblesse maybe added and combined with doing a lot of extra punch downs , the wine will be fine.