Maybe I've got a solution.
I've extracted all strings from KPW 5.3.3 recovery_util (initramfs binary responsible for handling updates) and found this:
ERROR:Refusing to update: %d update files were found. Expected to find only one.
What about creating yet another dummy update file when a full update package is detected ?
BTW, these two strings sitting next to each other look quite interesting:
It strongly reminds me The Answer to the Ultimate Question of Life, the Universe, and Everything (42)