With 5.1.0, the manifest of the apps is REQUIRED to contain "Extension-List" and related attributes.
See lines 24-26 of https://bitbucket.org/ixtab/ktfontha...a5f1/build.xml
for an example.
Note: the Specification-Version should probably be "2.0" (or even less) for downward-compatibility. 2.0 is KT Firmware 5.0.*, I don't know about older Kindles. For this particular software, I set it to 2.1 specifically because I DO NOT want the software to work on FW 5.0.*.
As far as I know, this is the only change that is required. I have successfully run fonthack on 5.1.0 by simply adding these attributes to the manifest.