For some reasons, La Fonera only allow users to upload a firmware released by FON.com. The /bin/fonverify upgrade script written by Pablo Martin is using a public-key cryptography for protect the firmware images. Each firmware image has a RIPEMD-160 message digest for verification. Only the verified images will be wrote into the flash.
However, since La Fonera is OpenWRT-based, you can do anything on the device once you get login into the system. There are several ways to do so.