Oddly enough, I don't see the evil clouds in any of the embarks I make (and I have been embarking in a lot of evil areas lately for this special tree farming business). Mostly I just get spontaneous zombies, evil rain, or both. The worst I've ran into are flying zombies and the muck rain that makes dwarves fall unconscious *shudders*. But that is a pretty good idea so long as my miners and masons don't get torn apart by zombie owls or something in the process.
Anyway, I did do the sapling gathering thing and that indeed worked, though glumprong saplings seem way more rare than feather saplings. This bug really makes things difficult to try and farm glumprong for a lot of reasons (like glumprong not growing in all evil areas, saplings being very rare to spawn, requirement to send dwarves into evil areas, and plants in evil areas usually dying faster than they can be collected) so I hope it gets fixed eventually.