As far as I can tell the issue here is with the BusyBox 'rpm2cpio', so using the GNU 'cpio' (i.e. the 'cpio.tcz' extenision) would not make any difference.
I had a bit a look into the BusyBox source code and 'RealPlayer11GOLD.rpm'. I've now come to the conclusion that the cpio-archive in that file has been compressed with the LZMA method, which BusyBox 'rpm2cpio' does not "like". It could support gzip, bzip2 or xz, but not lzma (maybe someone wants to raise an enhancement request against 'BusyBox' for that).
I realised that there are other versions of 'rpm2cpio' available via two extenions (i.e. 'file-roller.tcz' and 'xarchiver.tcz'), but looking at the long list of dependencies for each of them I decided to not bother with them. Instead I downloaded 'p7zip.tcz' and was able to use 7z x -so RealPlayer11GOLD.rpm | cpio -idm
I would image that the 'updatehelix' script could be changed to reflect those findings. In which case a cleanup step should also be changed to rm -rf opt usr rules.log to take care of all extracted files from the cpio-archive.