For this case, consider a phone stuck in a bootloop (not fixable through data wiping or USB jigs) with no CWM Recovery and no adb or fastboot access. Only an SD Card, the stock recovery mode, and Qualcomm's QPST serial port debugging interface.
Relevant files available are a handful of NANDroid backups of the same phone (made previously via CWM Recovery) and an "update.zip" from the provider/manufacturer that contains an OTA update.
Goal is to restore a NANDroid backup using the stock recovery's "apply update from external storage" feature.
Update: I've dug around, and as far as I understand, my stock recovery mode requires properly signed .zip files. I've made numerous .zips up on my own using various scripts and toolsets I had found, and some by tampering with the OTA update .zip I had mentioned. Attempting to apply either of them results in the exact same output that follows below. (The only one going past that point is the original OTA update.zip, whose instalation dies after the message "file_getprop: failed to stat "/system/build.prop": No such file or directory", but this is certainly part of the bigger problem with the device)
-- Install /sdcard ... Finding update package... Opening update package... Verifying update package... Instalation aborted.
Is my case hopeless without having the manufacturer's private key on hand? Can you think of any workarounds or alternatives? Could I be making a mistake in the zip-making process?