I guess what I'm asking, because I don't know, if I added a tree population that entered into the tgd as disabled while a render was in progress, would it effect/crash Terragen? As far as I know, there's no way to do this other than editing the tgd file via a text editor. But that would require a lot of work in forming the complete population entry in the text file, which would be a lot of work (though I know I could do it -- I spent a lot of time learning to parse the tgd file format.) However, I don't know how Terragen (the application) would respond to an update to the tgd file while a render is in progress. I think Matt might have to answer this.
Also, I have crashed Terragen by making changes (inadvertently) while a render is in progress.