I'm not sure I understand the concern about open carry. The intent isn't (as I understand it) to invite the great unwashed masses, but first responders. Now fireman and EMS folks may not be comfortable around guns, but I would expect most law enforcement folks to be comfortable, and if not, the point was (again, as I understood it) to start to establish that comfort for them.

If they arrived and left because they decided they didn't like citizens carrying guns, they probably aren't going to change their mind regardless of what we do.

The idea was to thank them, and at the same time subtly do a soft presentation of responsible citizens who are also gun owners.