Well, a will is a legal document, so the person the account is left to has that on their side in order to bring to Jagex as proof.
I highly doubt Jagex would refute or refuse the last wishes of an individual leaving something to a friend or family member.
Also, it is not like Jagex would really know if another person continually began using an account if it was not shared prior to the death of the individual. It is not as if Jagex thought I was hijacking an account when I moved from Texas to California. My IP address may have changed, but that doesnt mean I am not still me. They would not know the difference.
It's not legally yours, so a legal document in relation to this is foolish if you think it will hold any weight. That's like going out and renting a car, knowing that you have very little time left to live so putting that car in your will. It's not legally yours so you can't legally transfer ownership of it, because you never owned it.
And I don't know, I think it would be pretty sick (Not sick in the "Cool" sense, just disturbing) to be playing on the account of someone you knew who has been deceased..."Hey Tom!", "Oh no Tom died, I am his son..."...Awkward moment?