Description: The ability to add something to memories as you're writing it, from the full update page. The option could be underneath Backdate Entry, with a space for the keyword. Not as complex as the actual add to memories, but someone can go back and change the name that it was under if they want to.
Benefits: nothing earth-shattering, it would just make things easier. My first thought was that it could make things more user-friendly for people who use memories a lot, or who may start using memories because of it. bands who use LJ could categorize their postings into appearances, concerts, ect. and make the memories similar to a table of contents for their page.
i'm not quite sure of how it all works, but the main thing that i can see being a problem is the fact that an entry might need to be created before it gets saved as a memory.
the other thing would be that if it does get implemented it would effect the clients and such... it might be best just to do it on the web client so that if you have an entry that you know you want to set as memories you just update it from the web client.
Suggestions for implementation
i really don't know. if it's a lot of trouble to do it on a client, another option would be to have an option where people can chose to add it to memories after it's updated. kind of the same way that after you add a friend you can chose the group and color, on the success page when it's been posted you can have a link that says "add to memories?" or something along those lines. but again, this would only work if you update on the web.