Maybe so, but it would be pretty shady business to try to hide it when you know it's against the rules.
I agree with you. I don't necessarily agree with the rule, but I don't own or plan to own or breed Friesians. And I don't think I could purposely be dishonest. Especially if I know that the offspring would be registered with another association, for example, part-arab. All someone would have to do if they suspected someone of outcrossing is look in datasource. I would be nervous and paranoid all the time!
I wonder why the reasoning still stands in the first place though. I understand, originally, it was to help preserve the breed, but now it just seems a little protectionist. If you bred a Friesian stallion to an outside mare or vice versa, it doesn't make them any less a Friesian. Are they still that rare?