I feel like a don’t-notify character, no matter how well it’s documented, would realistically be kind of a hidden feature. Power users would make use of it but the average forum-goer might not ever discover it.
Clicking the generated link to see an option would be pretty hidden too. If there was going to be a UI affordance, it would be nice if it improved discoverability too.
Perhaps an icon on the generated link to encourage a click. Or a popup upon selecting a name from the autocomplete list. Or mcwumbly’s idea from above:
…an element similar to the address list in the direct message composer that would make it explicit which people and/or groups are going to be notified:
