I vaguely remember participating in such a thing years ago for single girls.
I think that it can be a great idea, but it has to be organized right in order to avoid the pitfalls mentioned above. The one that I went to was organized by someone who is a very good organizer and had previously organized several singles’ events.
I don’t remember all the details, but I think that there was a large group of girls who all met at a restaurant, and each girl spoke to each other girl and asked her what she was looking for, and if she thought of someone for her she let her know, and she took notes in case she thought of something later on.
It was kind of like speed-dating, but with all girls, and more informal. I don’t think Loshon Hora was an issue, since the conversations were one-on-one, and people were careful.
I don’t remember if she had shadchanim present or not, but it can be helpful. That way, if someone has an idea, they have someone to turn to with it.
I think it’s important to make sure that all those present are a certain caliber - people who are makpid on shmiras halashon and who aren’t superficial - that way it would be easier to avoid some of the problems mentioned by others.
I agree that it would be best to have a Rav involved and to make sure that everyone is in the same category. It would also be a good idea to tell/remind everyone of the pertinent halachos of shmiras halashon before starting.