I would recommend that you contact Artefactual Support to discuss what hosting arrangements can be established for such a feature before proceeding. In the meantime, some initial thoughts and resources:
You can find Entity Relation diagrams of AtoM's database structure on the wiki, if you'd like to explore:
However, I would strongly urge you to consider keeping such functionality out of AtoM, and either look into incorporating an existing third-party embeddable comments/forum widget, or developing your own standalone lightweight version, and then integrating it with AtoM, rather than trying to add this functionality directly. This will make maintenance - particularly future upgrades as AtoM's database schema changes - much easier, and while I can't speak to what our hosting team will and won't support, I suspect that something that requires far fewer code changes in AtoM itself is more likely to be allowed within your hosting agreement.
I think something like this would be the easiest way to add this non-domain-specific functionality to AtoM without having to significantly modify AtoM's existing database schema, making future upgrades much more difficult and labor intensive.
If however you do decide to try modifying AtoM directly, then I would suggest developing a custom plugin, and using a database migration included in the plugin to insert your own new db tables into the schema, rather than trying to repurpose an existing database field. Again, it would likely lead to less conflicts and challenges maintaining the work over time if you do everything you can to keep a clean separation between concerns. You can look at our theming documentation and slides for some of the basics on creating and registering a plugin, and the Development resources on the AtoM wiki include links to the Symfony 1.4 development resources as well. See for example:
I know of one other AtoM site that has developed something similar to what you discuss, though it also goes far beyond, in that it requires registration, includes moderation options for AtoM staff users, and even allows public upload contributions. See the Five Hundred Year Archive: