The problem I see with having a Parkhurst device attached is not in the use of the device.  For example:  If there were only 100 original devices made, and 250 people showed up with them, then 150 would obviously NOT be "As issued."  (yes, I know... these numbers are for example only)

Now I'm not saying that 5 or 10 shooters with Parkhurst devices would not be allowed as "As Issued"... just as someone with a replacement handguard or trigger assembly wouldn't be a disqualification.  However, what I am saying is that all Krags have a hand guard.  All Krags have a trigger assembly.  NOT all Krags have a Parkhurst, and the odds that 30 or 40 shooters would show up at a single event with an actual Parkhurst device installed would be extremely slim, given that so few of the actual devices even exist!