Hi Martin,
If you are looking for a .wis example that modifies installed files, extract vboxadd.wis from the VirtualBox Additions wpi on your ArcaOS install media.
This .wis uses cube to do the file edits because cube is always installed by ArcaOS. However, you can always write your own REXX script to do the edits.
Since you are newish to REXX, I recommend learning to use the TRACE statement and the RXD utility. They will make debugging your script go faster.
Regarding the SOM DLLs, do not put the 3.65 SOM DLLs that backlevel the versions shipped by ArcaOS into the .wpi or you will be unhappy with the results.
Since you are supplying a toolkit for building the distro, rather than a distro. I would recommend using WarpIN, rather than rpm, only because more users may be able to use the toolkit.