Cool, so I wasn't such an idiot to begin with.
I just tried something, didn't work but at least RCO editor didn't spit the dummy with errors. I pasted via Hex editor, a whole PSP RCO file infront of the PS3 RCO code. RCOEdit opened it fine, but only showed the PSP files. Like arlyn states, getting the original dev whom created the psp uncompressors, would be handy.
I'll keep looking for just the meat of the PS3 RCO file.
Also would it be possible for a Dev to upload another RCO file. Hopefully something like the music_plugin.rco, photo_plugin.rco, video_plugin_videotoolbar.rco, savedate_plugin.rco. Reason why I ask is that these are some of the PSP RCO and I wonder if the PS3 has the same ones name wise (of course with the different header, but they might be even more similar). I can post the whole PSP .RCO files used (i mean the file names) if anyone wants me to.
Dlna.rco and friendim_plugin.rco are not on the PSP.