That's probably the biggest problem. But I'd give it a shot and see how it turns out.
I would ask PG to change the TOS of hacker news, to allow you do this. I think he'd like it, so he may be willing.
Comments add value to a site, making it attractive to advertisers. That's the exchange. Asking contributors to also allow the site owner to re-use their contributions for other purposes crosses a line for me.
> And what effect might this have on the people willing to comment?
You incorrectly (to my mind) assume that this is a major problem for most people. I think there won't be any effect. People don't read T&Cs, though we make it explicit in them that we do this.
This isn't like Facebook changing their T&Cs. This is publicly posted information, not private correspondence between friends. That would be very different.
> Comments add value to a site, making it attractive to advertisers. That's the exchange.
That's not the exchange, just your perception of it.