Ah. Unfortunately, not that sneaky after all. Consider:

{
    package Test;
    use base qw(Class::InsideOut);    
    sub new { bless [], shift };
    my %foo : Field;
};
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%foo might now be tied but, since there is no way to access it, no magical attribute generation can happen :-(

Only works if you need to access the hash elsewhere in the same class - which may not always happen.