I've never had to deal with that, but I don't see any problem.  As far as I can tell, you are just certifying that 6 years have been done, not that 6 years are being submitted with the form.  

I suppose that if the taxpayer has a copy of the 6th year you would include it for reference (mark it as "for reference only"  and "already filed, do not file again".   But I don't think that would be necessary.