Should I submit a feature request to increment the preference on successful use of a custom daily deed?
Define "successful" for using an item that doesn't have a built-in preference to track it. For example, you mentioned the heart of dark chocolate. According to the Wiki, if you "use" this item, one of two things will happen:
You squeeze some fudgey blood droplets out of the heart and swallow them. It's as refreshing as it is terrifying to behold.
You gain 40-50 hit points.
is a "successful" usage and
You know what they say: You can't get blood from a stone, and you also can't get fudge blood from a fudge heart you've already squeezed once today. Wait until tomorrow.
is an "unsuccessful" usage.
Without built-in code to deal with this specific item, how can KoLmafia tell if "using" this item was successful?
Now, answer that question for some unknown new item that you can use two times a day. Or something with "zap wand" semantics that you can use a variable number of times per day.
We are happy (modulo a developer spending her own freely offered time to implement something she don't use) to add "built in" preferences to track new items. You can write a "New Content" request to mention such items in the Bug Reports part of these forums - or a "Feature" request, for items that have been around a while without being supported.
But a "Feature" to have Daily Deeds automatically track custom items and custom preferences and such will be rejected.