Thanks Norbyte, the updated version worked great!
For future reference by others, here's how it worked: in Blender 2.68, when I impoted COLLADA produced by your tool, I checked the "Import Units" option, and when I exported COLLADA, I checked the "Selection Only" option with the armature and mesh selected and left all other options with their default values. Then I ran your tool to export to GR2, which produced a few warnings, but worked anyway. Then I made the folder ..\Divinity - Original Sin\Data\Public\Main\Assets\Characters\Players and dropped the GR2 files in there, and now they appear in-game.
Thanks again!