I have successfully copied these 3 sprx files from DEBUG (DEX) FW 341 over to my retail PS3 unit running OFW 341 using dev_blind and Cinavia seems to be now disabled. I've confirmed this by playing back a number of Cinavia affected video files.
This means that Cinavia is now disabled for all DLNA playback and copying, but Cinavia checks on AVCHD and BD playback are still present with Message Code 3 that mutes the audio as the sprx file with the Cinavia check bdp_plugin sprx is not present in DEX firmware because BD playback is disabled in DEBUG. If someone is able to patch Cinavia out of bdp_plugin sprx this will be solved too.
I have created a new task for PS3MFW so this can be easily made part of any new MFW's as needed. Any FW version that has had the DEX firmware leaked should be able to be used to create a CFW of the same version. eg DEX 3.41 -> MFW 341 or DEX 355 -> MFW 355 etc etc. Don't use sprx files from different FW versions! See git hacks for change_cinavia_files task source.
Big thanks to the KaKaRoTo and REBUG team for their community contributions as this would have never been possible without the knowledge they've shared with us.
PLEASE NOTE I DO NOT OFFER ANY GUARANTEE WHEN MESSING WITH YOU PS3 SO DOING ANYTHING TO YOUR PS3 IS AT YOUR OWN RISK. I WILL NOT BE RESPONSIBLE IF YOU DAMAGE YOUR PS3
Cinavia files for 3.55: HERE
Cinavia files for 3.41: HERE
More information and discussion here: PSX-SCENE
A BIG THANKS to bitsbubba for uploading the Cinavia files for 3.41 and 3.55!