So this is mainly to share a recent observation and post some food for thought.
I've had a watermelon from my garden sitting on the counter since the first week of September. I just never got around to cutting it up. A couple of hours ago I hear "boom" coming from the kitchen. I didn't taste the resulting mess, but I did smell a pretty strong butterscotch aroma. Seems pretty clear that there was some sort of fermentation going on.
So of course the brewer in me instantly gets an idea. I'm thinking you could drill a hole that would hold an airlock, then use a marinade injector to pump in a yeast slurry into a few spots in the melon. Not sure what the results would be, but it would be a cool experiment. Maybe add some sugar or honey to boost the ABV a bit. Anyone ever try something like this?