With your eastern red cedar I had the impression you were actually in the zone where they grow (like "The East.") The couple of woodworkers I know are in the east; I doubt they have many contacts here in Texas.
I wouldn't try to plant a Red Cedar here in Texas. It would be a reminder of home only if it could grow happily, but they need way more water and probably cooler temperatures than this climate offers.
In their native habitat there are lots of very big trees with hollowed out centers. It's the nature of the tree, apparently; or at the very least, they survive for hundreds and thousands of years with this condition. Is this why you think it is unhealthy?
Would you post photos of the trees, and get closer to the trouble spots so we can see what the weaknesses are?
Here's a link to the hollow cedar that is in my family history:
("Slim" Husby is my great uncle Olaf). This photo is from flickr, I don't know who the people are.