Interesting...here's a post that mentions a branch of optiboot that will work with the Mega2560 board:
https://forum.arduino.cc/index.php?topic=368451.0
At the bottom of that post, "majek" claims to have a branch that works with the 2560...:
So....will that really work? I grabbed a copy of the "m2560" branch from github, and was able to build the hex file okay.
I'd prefer to use optiboot if it would work. Otherwise, I'll ask my question on the stk500v2 loader.