What is the best type of solder for strength and creep resistance?
While there are lots of solders available, none of what you seem to be looking at are remotely suitable - they'll solder copper just fine, but platinum is a different matter. You need to start looking specifically at platinum solders. Since the melting point of copper (1085 C) is an obvious limit to your process, you should be aware that only the softest platinum solders melt below this temperature. While it's true that only local heating of the copper is necessary, this isn't much help due to a) the high thermal conductivity of copper, and b) the fact that you'll be heating with a torch or something like it. 

Frankly, I'd suggest you look into spot welding or something similar.