Your looking for a very specific target availability and your looking for it to exist in a place where the users do not exist meaning your users come from a forum not our product right?
I do not think something like that would be theoretically possible to be some standard inclusion in a forum integration. Your talking about we would have to query the products table group the results by userid and then issue a query for every user to get there email from the forum database. That means in your case 420 extra queries. That is because the forum and gallery products do not need to exist in the same database.
The only way to do it for it would be something included in our product is for it to be one query and I can definately create an addition case to email users whom have an ad in the standalone classifieds mailer. That would be easy.
Do you understand what I am saying here? I am not going to code something in the application that would take 400 or 1000 queries to complete. Talk about bringing a server to its knees.