Git Fast-import does not support metadata properties so there was no way of migrating these infos solely with the existing tools.
Since there was no response and losing all these bug-ids was no option for us, I wrote a fast-export stream-rewriter which is capable of rewriting the stream in a format that git understands but with all our bug id infos included.
The C# rewriter works by executing the following steps:
- Parse commits from input stream
- Check if a commit contains a property bugs command
- Parse the bug id and modify the commit-message with the extracted bug id
- Exclude other unsupported bzr-fastexport only commands ("feature", "property branch nick"))
More infos about this rewriter + the code can be found here: http://www.fusonic.net/en/blog/2013/03/26/migrating-from-bazaar-to-git/