How do I convert a NANDroid backup into an update.zip file that works in the stock recovery mode?

How do I convert a NANDroid backup into an update.zip file that works in
the stock recovery mode?

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.