Is it valid if I mark a bug "Fix Committed" if the package in question is hosted upstream (outside of Launchpad)?
You can find a list of Launchpad bug status definitions the way they are used in the Ubuntu bug tracker at https://wiki.ubuntu.com/Bugs/Status. Normally, for Ubuntu package tasks, "Fix Committed" doesn't correspond to "the fix is upstream but not in Ubuntu yet"; for that you can usually just set a bug watch and let Launchpad deal with it, as Jorge suggested.
Note that some Ubuntu teams assign different roles to certain Launchpad functions, including bug status; this can initially cause some confusion. If you're consistently triaging bugs in a particular component, it's a good idea to familiarize yourself with the way the team responsible for that component does bug triage.
You shouldn't need to do this by hand. If you set a bug watch Launchpad can monitor upstream bug reports and set the status automatically. For this to work the project upstream needs to have it's bug tracker registered in Launchpad. (You can always add more if it's missing)
When you do this Launchpad will then track the bug in Ubuntu and upstream. That makes it easier for developers to find bugs that have been fixed upstream and not yet fixed in Ubuntu.
In your specific case wajig didn't have it's bug tracker registered in Launchpad. I went ahead and did that. Unfortunately the project in Launchpad isn't set to use the external bug tracker. You can probably contact the person who registered it to change it so that it points to the actual upstream wajig tracker, which will make it easy to link bug reports in the future.
Since that STILL doesn't solve your original problem I left a comment in the bug report; which will hopefully point the right people to the problem.