Thanks again for the interesting reading, but I'm going to go ahead and end the thread because of what I learned.
For those who don't know, saguaro's have a ribbed infrastructure for environmentalists pleasure.
So when the meat goes away, long (or short) ribs are left behind, but they are not really large. Pretty thin. I thought if I could find some of the old weathered ones and new ones I might see if I could make them in to a hiding place and put plants on them. But it turns out there is not really enough rib to do anything with.
Then...I thought cholla would would be perfect. All the holes. And that turned out terrible. I boiled two samples. A "fresh" piece of wood and and old weathered one. The old weathered one almost turned to mush when being boiled. The fresh one, which had dried cactus meat still stuck to the inside (but was all very much dry), was boiled for 8h and still floated and I had to scrape all the meat off. Also, a lot of the cholla parts were mushy. I put it in my testing tank, and it floated. I could boil it for 24h, but I'm going to call the experiment a failure.
Besides, boiling cholla wood smells like cat pee.
And yes, grapewood rots over time. But very very slowly. I've used it as a framework to grow roots and rhizomes in shapes, so it has its uses. Just limited uses.
So this experiment is over, I will only worry about it again if I find a saguaro rib that would actually look good. Only good looking wood I have on my property either is not useful for the aquarium or is petrified. Which looks great in there. If citrus wood was safe, I'd have some awesome branches I could make in to "stumps", but so much leaches out of that stuff. So thanks everyone.