Blender COLLADA 1.4 physics import/export python script is being developed by several developers (it is an open source project). Often the developers focus on graphics or animation features, and break the physics features (they don't test physics export/import). If we could use COLLADA DOM inside Blender, it would be easier to guarantee a working round-trip (export/import).
This is not a Bullet issue but Blender issue (related to COLLADA 1.4 python support). So if you want to improve the Blender COLLADA Physics support, it is best to report those issues in the Blender bug tracker:http://www.blender.org/development/report-a-bug/
Thanks for the feedback,