I'm quite disappointed to see that the UK-based wikimania team seems to have decided that everyone will have to get a credit card to be able to participate wikimania. A few years back this would indeed have stopped me from participating in Wikimania. Paypal was already a huge step for me to take - credit card was simply impossible.
I'm also very sorry to see that this is only surfacing now (why didn't you ask people about this? Missed opportunity), the evil of assumptions is indeed valid, the evil of using the organizers' ease of administation as starting point maybe even more.
I would like to join the chorus to allow paypal, but i would also strongly suggest to open a separate bank account for wikimania, and allow people to send their money there, using the invoice code you provide them in the payment instructions as a description. I'm not convinced that mixing it with the large amount of donations would actually be helpful (that is asking for mixups etc).
However - I do remember how complicated it is to set up a payment system - even one like paypal or iDEAL that seems simple but isn't that really (you need to track payments and confirm the payment made is matched with the registration). So given the short time, I think the most reasonable would be - even though quite uncomfortable - to have cash on-site payment options. Everyone making it to the conference will be able to handle that, the big downside is that there are some security issues there too. The good news is you still have two months to figure those details out.
Best,
Lodewijk