That is pure myth. Everyone was crucified the same way no matter who he was. One might just as well reason that the one who wanted to be crucified upside down just wanted to be different, to stand out from the others who were crucified normally. So rather than a symbol of humility it might be construed as a symbol of pride.
A gruesome death by suffocation occurs in a normal cricifixion, but how does one die upside down? Wouldn't he pass out first.
BTW Thanks for the link with all those crosses --- amazing. Every religious order and group had their own distinctive cross --- obviously a lot of cross-breeding going on in the church at that time --- but very few upside down crosses.
Going for 2000 posts :)
How can you say that it's a myth? Do you claim that St. Andrew's crucifixion story is a myth too?
I trust though, that your fears about the Catholic Church creating a Satanic symbol have been allayed.