I have tried Toolkit V6, but the "OPOS 104 virginiser" doesn't work on these cards. I think the cards may have either been programmed with the old 103 -> 104 upgrader, or started out as 104 cards. Either way, they don't have the erasable 104 OS installed.
I did notice Colibri's TitaniumHack tool could convert from Titanium2 to Opos104 - so an SDK for Titanium2 may work too. From what TitaniumHack does, I get the impression the only differences are encryption keys and header formats.
And yes, people are still using these cards, just for different things. I had some Titan2s in a drawer and these Opos cards came with some Funcards I was using for Hacktv (https://github.com/captainjack64/hacktv).
I have the Opos 104 OS BIN thanks to that kit, if I can figure out the memory map of the 6464 card and where the OS loads, then I may reverse-engineer the Titan2 and OPOS operating systems and build my own AVRGCC-based SDK.