Strangely DMM.co.jp accepts Paypal and Bitcoin to be converted to 'DMM points' which I think (?) can be then used to make purchases:
https://point.dmm.co.jp/choice/pay?basket_service_type=my
I do know Paypal does not allow their services to be used for digital adult (with a few exceptions such as the arrangement they have with Epoch.com) and I don't have the time to read the DMM fine print regarding Paypal so it may be only possible to use those points for digital non-adult and physical adult purchases.
However, they take Bitcoin too there so perhaps it's only a matter of time before that option comes to R18. Consider emailing them or posting in the sticky above.
And sorry to be kind of ignorant here, but what are the hurdles to getting a credit card in your part of the world? What about a pre-paid one?