Very difficult to do an editable flora version... without original source, anyway.
Pretty difficult even if you did have the source, but I agree, would be a massive upgrade.
Even nicer - setup the whole scene in Vista, then export the whole shebang to Lightwave for rendering.
Then perhaps a fauna editor, for doing six legged alien cows or similar... OK, now I'm going off into wild fantasy land. I'll stop now.
About 14-15 years back (egad!) I wrote the author and asked him about the possibility of doing an updated version. He said he was always tinkering with it, and even sent me a couple of renders he'd done with an early build that did grass. Didn't look very good. His suggestion for undergrowth like that was to shrink trees down or set the treeline variable so that the trees just poked up above the ground.
Honestly, I have never ever seen a 3d package that was so intuitive to use. Everything you need in it is just...right there. I tried Bryce and other tools back in the 90s and early 2000s and to be honest none of them were worth a damn. Bryce's interface seemed to go out of its way to be obscure and confusing (for example).