Could any Peter Cushing footage in another movie suffice to add Tarkin to the Prequels even for a cameo as an Admiral, perhaps on Anakin’s battalion? Could one incorporate an idea of him being the one to design Vader’s suit?
I could survey his Frankenstein movies but I’m not really sure if one could use material from it considering he spends the entire time in Victorian era fashion.
Your best shot would probably be to deepfake Tarkin onto another character in the background.
But as is, for me the cameo in RotS is enough. And he also shows up in Clone Wars, so no urgent need imo.