I just bought a 2010 RAV4. It came with 2 remote keys plus a valet. The dealer tried to get me to sign up for "lost key protection". It was a lot of money, like $295. It came with the engine interlock (chip). Is there a way get a spare full-function additional key that is fully functioning with my current car without having to go to the dealer? I'd be willing to send in the second key if that would help in any sort of cloning that might be required.

I'm afraid not. It must go to the dealer. If it were that easy, vehicles would be stolen left and right.
You can buy a cheaper key, (no fob attached) and have that programed as a spare emergency key. The dealer must have a master key, the vehicle, and the Toyota scan tool to program.