Hi Gustav, I think Ma Huan did know the higher society, he just didn't get to inspect higher ranking keris. Probably he sees lower officials keris. People at his level; people that he can be easily approach and ask to see their keris.
In the code of Melaka (probably around 1450s) it is stated that keris with gold hilts were prohibited to be used by lower ranking people unless it is a gift from the king. I don't know how the people at that time divide the ranking, but a later document shows that the ranking used in a Malay kingdom is quite complicated and probably derived from the caste system.
Apart from the clear cut kshatriya, vaisya, sudra etc they also have people in middle ranks. When a kshatriya married a vaisya, sudra etc. If i remembered correctly one of the middle ranks are called magadha (Megat in Malay) and there are other lower middle ranks also. If the Malay ranking system are derived from the caste system, I think that Majapahit also would have something similar. So perhaps Ma Huan meets these people? Again, if it is indeed a keris. Or probably he just sees a badik..
But come to think about it, this would confine the usage of luk keris to a very small group that is also can be argued.
On ivory, I am not aware of any prohibitions of using ivory for lower ranks/commoner whether in Majapahit or Melaka.
p/s: just saw Alan's comment on varna. Changed it to caste.