Happy to do this over beer, but I'm not sure I understand which of the SCI rules would determine which CA you use.
I'm a little amused (not at you, but at the absurdity of the whole process) that we're talking about CA selection for products that almost definitely can't even properly quote and unquote a FIX field separator from user input.
Whether it's ceremony or not, I have to check the box or face harsh regulatory penalties.