It's on the road map, but not with any set time. It's one of those things we will probably do when we get some funding for it, eg. when someone needs it and is prepared to pay for (or at least contribute to) the development costs. The way we usually handle that is if it's something which would be of benefit to Fabrik and we'd want to add as a supported plugin, we split the hours.
Nope, no instructions on coding it as a custom PHP submission plugin. Writing instructions on how to code it would probably take as long as actually coding it.
Somewhere I do have the code for a working PHP submission script that uses Authorize.net, which uses broadly similar principles. But it would take some effort to dig it up and publish it, as it's on a client site (so I'd need to get permission, and sanitize it to remove some proprietary parts of the payment process), so it's not something I can afford the time for in Community support. That's Pro level stuff.
-- hugh