Pak Mudi, i commend you for your desire to teach and inform the world about Indonesian culture. Frankly i would much rather hear about your history from native Indonesians than from the Dutch, though it is true that they probably have the finest keris collections in the world in their museums. But the history of a people told by their conquerers and colonizers can often be misleading.
I think that pusaka has everything to do with power. On a royal level it deals with the power and right of rule amongst othe things. On lesser class levels it deals with the power of a kinship group, passed from generation to generation.
The Mona Lisa is NOT pusaka. One might look at it as a national treasure, but as you point out, which nation. DaVinci was Italian, yet it resides in the Louvre in France. I think at this point it has become such an icon that it can be considered a world treasure. But it is not the heirloom of any kinship group and does not serve the same purpose in Italian or world culture (if there is such a thing) as pusaka.
When the royal regala is presented in ceremony and there is a procession of teh royal pusaka you really don't believe this is about power? Possession of the royal regala asserts ones right to rule. This has everything to do with power.
The word pusaka is used in a number of ways in the description of keris and other heirloom items. For some it describes the quality of the keris. For others it is more personal. I have heard some people argue that only royal and court pieces can truly be considered pusaka. Others will argue that keris on any class level that are owned and passed down within any kinship group also qualify as pusaka. I tend to favor this second argument. If a keris is sitting on permanent display in a museum is it still serving it's purpose as a pusaka for anyone's kinship group? We call it pusaka in respect for it's past history or the incredible quality of it's workmanship, but hasn't it's chain of power been broken?