Code:
[eschwartz@arch ~/Downloads]$ wget http://s3.amazonaws.com/G7G_FirmwareUpdates_WebDownloads/update_kindle_voyage_5.6.2.1.bin
[eschwartz@arch ~/Downloads]$ kindletool extract update_kindle_voyage_5.6.2.1.bin kv_update_5.6.2.1
[eschwartz@arch ~/Downloads]$ cd kv_update_5.6.2.1
[eschwartz@arch ~/Downloads/kv_update_5.6.2.1]$ gunzip rootfs.img.gz
[eschwartz@arch ~/Downloads/kv_update_5.6.2.1]$ mkdir rootfs
[eschwartz@arch ~/Downloads/kv_update_5.6.2.1]$ sudo mount rootfs.img rootfs
[eschwartz@arch ~/Downloads/kv_update_5.6.2.1]$ cp rootfs/opt/amazon/ebook/lib/ReaderSDK-impl.jar ./
[eschwartz@arch ~/Downloads/kv_update_5.6.2.1]$ sudo umount ./rootfs.img
[eschwartz@arch ~/Downloads/kv_update_5.6.2.1]$ unzip ReaderSDK-impl.jar
[eschwartz@arch ~/Downloads/kv_update_5.6.2.1]$ grep -R "/mnt/us/FONT_RAMP" com/amazon/ebook/booklet/reader/
Binary file com/amazon/ebook/booklet/reader/impl/J.class matches
Yay.
Edit: Yes, to be clear -- this is a step-by-step of me downloading the update, extracting it, and searching inside the right java package for the name of the hacks's file. Since a text string cannot be byte-compiled.
Finding it (I did) demonstrates that the hack should still work.