It appears that I have got a little bit behind with my blogging! This is a recap of a bit of learning, and publishing it my help me find the information again if I need it in the future.
I applied what I thought was the fix, and then had to get access to one of the porter boxes to test it. As I am not yet an official Debian Developer, this required writing an email confirming that I agreed to the Debian Machine Usage Policy. Then I had to sign it with my PGP key and and send it to Andreas Tille (my regular Debian GIS sponsor) to sign it and forward it to the Debian Request Tracker. The process is here for future reference. Then I had to fiddle around quite a bit to be able to email my SSH key to the machines network.
But eventually, I managed to start testing using a schroot on one of the armel porter boxes. But unfortunately, my fix failed! Due to my C++ skills needing some work, I called on the help of Peter Green who had sorted out a similar problem with QGIS in the past. He sent a patch, but suggested that the QGIS upstream project should probably check it.
This resulted in some more learning! The QGIS guys wanted the patch in the form of a pull request on Github. But once I had managed to do that, the pull request was very quickly reviewed and merged into QGIS. Then I was able to ask Andreas to sponsor an upload of the fixed QGIS to Debian. It is currently waiting for a slot for the gdal transition.