The extra hole on HD disks were designed to let a HD disk drive recognise when a high density disk was inserted as a sort of backward compatibility to allow it to also support DD disks. DD disk drives did not need anything extra to support DD disks (as that's what they do ;-)), and don't need to specifically detect HD disks, cos they can't support it anyway.
As a result, I imagine most DD drives neither know nor care about the extra hole, so that idea will most likely not work.
I think it's time to bite the bullet and buy the disks. You get 50 in a pack, even if it is expensive compared to standard disks.
Definitely better value than 8" disks :-o
