So now I know -- wouldn't have worked.
They took down the stump today, and it was just crumbly mush. Live cottonwoods are very strong, with plenty of sap/ moisture -- 15-year-dead cottonwoods are basically balsa wood. No way would it have been strong enough.
Which is a relief in a way since it's gone -- would've kind of sucked to find out we could've done it after all.
Meanwhile, the tree trimmer guys were rhapsodizing about the possibilities of building a treehouse between the two live cottonwoods -- we're doing it sometime, I swear.
For now, I'm focusing my "cool stuff for sozlet to play with" energies on using one of their ropes to make a super-cool swing for sozlet. The way the trimmers have been working is that they threw ropes WAY up there, and shimmied up them (a circus act in our backyard, amazing), and then cut from up in the tree, chainsaw-wielding monkeys. They're not quite done, and one of their ropes is still there. It's hanging from a branch about 50 feet up, and I thought wouldn't it be cool...? And we know it's a good strong junction. I'm going to buy some good rope this weekend, and then ask if we can attach our rope to the end of theirs when they're done, and pull ours through. And then have a 50-foot swing!
It hangs in the middle of the grassy area, plenty of room to maneuver. We can pull it out of the way to mow or whatever. Not sure if I'd want to go with a standard long rectangle of rubber, or something like this:
(Dragon made out of recycled tires.)
Love the idea, though.