Hello my little druid Tewin just figured out how to move while in Treeform and keep the hp regen.
Since I'm level 10 I'm leveling in Misty Thicket, so i find a spider and let it poison me, then kill it and cast treeform while still poisoned. You'll notice that treeform icon doesn't show up in the buffs area. Once the poison wears off you will be able to move in treeform and keep the HP regen.
Here is a video on how to replicate it:
http://sendvid.com/wfq1am56