I've been running the hosted JIRA for a long time now and also a hosted bugzilla at cloud forge. I recently went through all the hoops you ask us to to export the Bugzilla DB, stand up both a local JIRA and Bugzilla, imported the exported Bugzilla into a local Bugzilla, then used the local JIRA Bugzilla converter plugin, all went very smooth. Unfortunately that is when the pain starts for a JIRA in the Cloud user...
Cannot do a direct JIRA backup/restore (unless of course we want to wipe out three years of other projects...), tried the Excel export from local JIRA, open in Excel and save as CSV (somewhere in this process I believe the data was hosed... probably opening in Excel
), and tried importing that into JIRA Cloud... 7588 bugs in the local Jira, 13 in the Cloud JIRA after... So why won't you just support the Bugzilla plugin for Hosted Jira? Any other non-destructive ways of importing our 7000+ bugs into an existing Cloud JIRA instance? This can't be that hard to support, we've sent men to the moon and back. 
Thanks,
Joel