at the moment you are just able to donate with the help of other members, like sending them the paypal amount and they donate for you,
other members also accept rsgp as a payment method
Getting in contact with bond holders might be a bit difficult depending on how you'd like to donate. Dave is becoming fairly active again, and Moes is usually around. Black Tux never answered my pm's last year about donating so I can't vouch for him. Good luck donating
I would recommend your first time through the method of RSGP otherwise known as Runescape gold in case you're unfamiliar then I would donate via paypal after that.