It is NOT possible to incorporate custom content into an OS update by customizing a SIG file.  For Windows Mobile, the update mechanism (Microsoft Update Loader) that is mandated by Microsoft, does not support loading customized content and there is no way for us to circumvent that limitation.

However, we recognize that there are times when it is important to be able to populate content into the \Application partition in the same operation as an OS Update and we have provided a means to do so, although it is a different mechanism than that used for Windows CE and has more limitations.

For Windows Mobile, a CAR or GAR file can be created and included as part of the OS Update package.  When such a file is included, it will be unpacked and used to populate the \Application partition BEFORE rebooting into the Microsoft Update Loaded to perform the actual OS update.  This means that if the loading of the OS itself does NOT cause the \Application partition to be erased, then the desired content will be in the \Application partition once the Clean Boot is performed at the end of the OS update.

Whether the loading of a given OS will or will not cause the \Application partition content to be lost generally depends of the combination of the OS you are updating from and the OS you are updating to.  If the size of location of the \Application partition changes, then it will generally be necessary to reformat it, and hence it's contents will be lost.  This generally occurs when going from one major version of the OS to a newer (larger) major version of the OS since the \Application partition may have to shrink to accomodate the new larger OS.  In most cases, a minor OS update will not have this issue. The instructions for creating a CAR/GAR file and the requirements for making one work as part of an OS update are provided in the OS Update Package Builder documentation.