Thanks for your input.

I find functors a bit obtuse, and I've spent more time on this than I should...just brute-forcing it...

Code:
    uint16_t sum = 0;
    for (int i=0; i<byteVec.size(); i++) {
        sum += ((i&#37;2==0) ? byteVec[i]<<8 : byteVec[i]);
    }