I believe a subscription model demands a degree of consistency. You expect to get your money's worth each month.
Subscriptions are planned for magazines (Comic Kairakuten, X-EROS, etc.). They come out on a regular basis (almost all of them are monthly), so a subscription model makes sense for those.
H-manga are trickier. One month we might offer a single book, and five the next. I can also see the scenario of releasing three rape themed books one month, and three vanilla ones the next. I don't think it's a good idea to offer a service where customers might feel tricked if the paid for the "wrong" month.
That being said, it's too early to rule anything out.
As already mentioned, Fakku Gold was given (for 1½ month or so) to everyone who bought Renai Sample as an apology for the delays.