Hi.

The .upgrade file actually contains of many (and varying) chunks of data, kernel binary, flash root (about the function of initrd), root partition,...
Check the developer info on riocar.org and/or on my homepage at http://www.incase.de. Among the downloads on my homepage, you will find an upgrade splitter and upgrade builder which is hopefully up to date (i.e. supports the newer HW-REV chunks that define the hardware revision(s) an upgrade file can be applied to), if not: email me.
So basically: There is no constant offset for the partition images in an upgrade file, but the format of the upgrade file is documented, a builder/splitter for it exists, and the partition images that the splitter/builder outputs can simply be mounted using the loop mount option (offset=0 AFAIK, but the offset is not needed with those).

cu,
sven

PS: Still missing <table> from UBBCode