I am learning the file format. I have seen evidence that these ps3 rco files are showing structure signs of older <2.60 rco files. I will focus on the uncompressed rco files now as I believe I can uncover most of the structure rather quickly.
Dug up this old tool and got results using dlna_plugin.rco from xfcj's modified rco's.
C:\Unixdos>rcotool -l test2.rco
*.rco detected... fw ver: 1.50-2.50
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Both RCOEdit and rcotool are showing promise. This result may also explain why Ressursiklunssi does try to process graphics even if this file is not compressed.