Executives are more helpful than most give them credit for. If you're a credible contact with real experience, and authentically ask someone for input or advice, most are happy to chat.

The thing about the vaunted "HBS network" is that if you're qualified to get in to HBS, you're extremely likely to be otherwise credible; those who wish they had access to the alumni network also tend to not have enough meat on their CV to get access either way.

I personally suspect that the most important networking aspect of an HBS degree is just humanizing executives and learning how to communicate with them. If you've got those two things down, actually connecting is much easier.