Maybe ripening could be a feature of the fruit growths? It's not yet a feature of vanilla DF, but modding it can be done by introducing new growths that would come before the ripe fruits, to the beginning of the vanilla fruit growth time (somewhere around 120000:149999 or so) while the ripe fruits have the rest. Unripe fruits would be mostly unusable (I just make them use the same STRUCTURAL material), but a few like bananas and mangos could be used for cooking while unripe, but not brewing.
I see some potential problems with this approach with the current plants implementation:
Case 1. Unusable unripe fruits. If the fruit is unusable while unripe, this is functionally equivalent to just reducing the available time of the fruit. I do plan to reduce the available time of a number of fruits, but adding a "unripe fruit" growth doesn't seem to have much value.
Case 2. Wild plants with usable unripe fruits. This would provide double yield - a full crop of unripe fruits, and a full crop of ripe fruits. There's currently no way to have growths
replace each other - only occur in sequence. Picking bananas before they're ripe because you need food
now is interesting. Picking bananas before they're ripe and then having fully ripe bananas pop in
anyway is not.
Case 3. Farmed plants with usable unripe fruits. This would provide
lower-quality yield than vanilla. Farmed plants drop usable products randomly - you can't choose which growth to pick, so you would get half ripe plants, and half unripe plants. This is pretty much the opposite of how farming should be: your dwarves planted them in neat rows to keep an eye on them and be able to pick them at the right time with a minimum of fuss.