You are right, the arf file is just a tar file in disguise. The problem is that the arf file contains the whole file system for the Receiver including the /dev entries that are used by the device drivers. For the /dev entries to work you need a app that honours all the extra data that tar files carry concerning Unix files (in particular in this case the device major and minor numbers).

WinZip does not honour this info, meaning you cannot use it to patch the arf file.

It should be possible to use one of the Windows ports of GNU tar to patch the arf file, but I haven't tried it myself yet.

I am investigating providing other ways that people can patch their arf files.