Oh well, seems I was a little quick with this one. The exported file now has the proper format but however the values dont seem changed at all. Doing a compare between the bvh created by the regular exporter and the one exported by the script shows identical files.

Setting exoptions[17] = 0 again results in the file that was created earlier before your mod. Which again is the same as Poser exports if you disable autoscale in the dialog. So it seems all the changes you make to the offsets are discarded.

...please tell me this does not mean you can't save the modified animation? Maybe there is just a refresh missing or something like that?