Salt would do it, although I didn't know it was used in funerals.
So it turns out Akai and Okiya's preference for high collared outfits was not a matter of dress preference alone. He probably has a scar, tattoo, birthmark, or some jewelery that he keeps on him. The pictures of him in the past in 802 don't show anything obvious, so it must be something small or something that happened later that her sister informed her of.
Haibara had a lot of guts to go after Okiya like that. It looks like she barely hesitated when she saw he was off guard. This pretty much shows she thinks Okiya is not likely to be harmful, because I don't think she would risk the chance of getting caught by him while they were alone. Even if Haibara didn't get the chance to see what she was looking for, his block pretty much confirmed he has something to hide under that scarf. If I were her I would switch tactics and move to interrogating Conan about his relationship with Dai because Conan is a lot worse at lying under pressure and she has leverage over Conan. I suppose Haibara could take the cat-on-stairs tactic and try to trip Okiya so she can have a go at his collar.
I think she probably should have gone for his glasses though. She would have gotten caught, but the eyes would be a dead giveaway.
P.S. PLOTHOLE: How could Haibara really be sure Okiya was sleeping and not just staring off into space at his lap?