Yeah, getting a firm one is key like you said. I did one with just a heavy coat of coarse salt and let it sit for an hour before smoking. That really pulled out some water and let the smoke flavor get in there. I used cherry wood and it gave it this sweet, almost ham-like taste. It's messy to eat but so good.
Yeah, smoked watermelon is a total game changer. I tried it last summer after seeing a video, and the trick is to get a really firm, seedless one so it holds its shape. I did a simple rub with just salt, pepper, and a tiny bit of chili powder, and the smoke makes it taste like this crazy savory candy. It falls apart a bit when you slice it, but that's part of the fun. Honestly, it's the best weird thing I've ever made on my smoker.
It sounds like a fun experiment but calling it a game changer is a stretch. The texture gets way too mushy and the flavor is just weirdly salty fruit. Maybe it's just not for me.