I love the idea. However, I have to say that I agree with you that even with 100% watermelon juice I fear it won't taste as watermelony as you would like. This reminds me of the time Jerry wanted to date the roommate of the woman he was dating. He and George agreed it was an impossible maneuver but eventually came up with a foolproof plan ( the threesome).
So this is my brainstorming.
1. No extracts etc.: smells like perfume.
2.: How about reducing the juice by boiling down. Maybe 5/1.
3. Does the rind have any oils? You could put it all through a macerating juicer and add to tertiary.
4. You could dehydrate slices of watermelon and dry watermelon in secondary.
5. For acid, instead of just lemons add some raspberry or macerated cranberries. This will be a complementary flavor but also turn it more pink. I find with subtle fruit flavors that being the "right" color tricks the mind a bit.
6. What's the watermelon hops? El Dorado?
7. Serve with a slice of watermelon over watermelon ice cubes.
Good thoughts. I had considered the reduction, but I'm always afraid that you'll get "cooked" flavors that just aren't the same as fresh fruit. If the rind has oils, it doesn't remind me of the fresh fruit character I'm looking for. The rind always seems to have a "green" note that doesn't interest me. Same with dry hopping - I've tried it in cider and I'm not a huge fan because of the other hop flavors and bitterness that carry over.
As far as the berries go, that sounds like a good idea, but I think they would take over the melon too quickly. I like lemon here because it can give a citric tang without too much flavor impact. The dry-meloning sounds like a great idea if I can find the opportunity to dry out some melon slices.
The real winner sounds like melon cubes. It kind of fits the sangria vibe I'd hope to get out of this. I'm also thinking of backsweetening with more melon juice, since I'm sure this would end up bone dry. This is sort of how I make sweet ciders - target a higher ABV than where you want to finish, then backsweeten by diluting with fresh juice.
I really wish I could sample the brix on melons in the market. I've heard that they can range from 10-15 brix. Obviously, the super-ripe ones would be way better for this application.