Here's an answer coming in from Ann Marie Carlton who works with an Atmospheric Multiphase Chemistry Research Group at University of California, Irvine!
"There are several examples in the scientific literature of biochar adsorption for ammonia. I found one manuscript that described removal of H2S with biochar in a digester. I think biochar could work for ammonia in a straightforward way. The time to breakthrough (when it stops working) would depend on a lot of variables: ammonia concentration, pH of the char and relative humidity of the air. For H2S removal, it's less clear that it would work in an easy straightforward way, but maybe."
@stevie Wow, that is exciting. I'm almost picturing pomander balls of this stuff hanging around.