The IPS Suite comes with the above per-group option. Unfortunately, when enabled, the setting is applied throughout the whole community, which means I can't restrict its effect to a specific forum or topic.
In short, what I need for my community is a mod that allows me to limit the number of posts that can be added to a specific forum (or even better, a topic), by each member of a specific usergroup.
Additional details/requests:
I need this to be a per-group setting.
I'd appreciate a counter above/under the editor, which displays the number of posts that can still be made by each member of the usergroup (if a restriction is applied).
If you're a developer and you plan to add it to the marketplace, I'm willing to fund the initial release of the plugin (of course assuming the cost will be within my budget).