You can do this sort of thing, but right now you have to handle it at the application level. Essentially when deciding on the "final" characteristics of an asset, you would iterate over the custom fields from each of the issuances given by listassets, and overwrite any values with the most recent version.
We did think about doing this in MultiChain itself, but felt it didn't fit the paradigm of a blockchain as an immutable append-only record of transaction history. In other words, the old values of those fields will always be there in the blockchain, so it seems more sensible to make those values explicitly visible through the API.
Though maybe in future we'll add some output to listassets which does this summarization for you.
