What about areas where parts of transactions are displayed to humans? For example, the name of a self issued currency ("BIGDIK" in my example). What about phone numbers, URLs, or other messages (e.g. "Litecoin is crap") appearing in description fields?
I don't think it much matters because you can't easily get a human's attention. Most likely, nobody is going to look at those fields. We've tried to keep the design so that you can't easily compel another person's attention.
If models do arise, I think probably the best solution would simply be for the client not to report it to the user. For example, if someone not on your list of known accounts sends you a microscopic amount of XRP, you probably just don't want to get notified. If we do things like integrated messaging, they'll have anti-spam features built in such as requiring permission first.
We are not really expecting the transaction fee to protect against that kind of spam. It may to some extent do so "accidentally", but that's not what we mean when we say it protects against spam -- we mean pointless transactions aimed at clogging out legitimate transactions.