For myself, I’ve never really tried too hard to do this, nor found the need to. I find that the types of things I use Scapple for are more along the lines of what I would use a plain-text editor for. Simple and often disposable staging grounds for things which may evolve into more concrete and permanent structures, but if done so, nearly always in something else. I don’t find that the way in which Scapple promotes thought is all that conducive to forming a foundation of ideas in an environment that is tailored toward making something (rather than just thinking about it).
So when it comes to making that leap from ideas to building, I don’t really want the scraps, false-starts and useful nuggets in Scrivener (or whatever program I’m using to manifest the idea) as native entities in that software. Sure, I want that board in my Binder so I can easily refer to it and maybe even add to it by opening it in Scapple, but there isn’t anything in the Scapple board itself that I want as outline elements in Scrivener.
There is one exception to this very formative use of Scapple, and that is in the composition of short pieces of written material—such as correspondence like this—using the Stacks feature. Something like this post could be expressed in a per-paragraph basis, with rewrites in separate columns. Then I would select the final version of that and copy and paste it into a text file or browser field. That’s probably the most pragmatic use for Scapple that I have. I like to use it as genuine text editor that lets me write anywhere on the “page” and easily sideline tangents without outright discarding them. And yes, sometimes these end up in Scrivener, but always as a singular text file in that case, and always as a one-way flattening of the Scapple idea space into a solidified and linear form.
This all probably stems, at least in part, from my philosophies on the development of ideas, and how integration, synchronisation and other conveniences are ultimately detrimental to the process. I would rather hand transcribe notes from a paper journal than use a digital journal that “just works” everywhere I go and automatically dumps content into my computer through “apps” and “clouds” and all that. I would rather mark up a static file and then review those markings one by one, than have edits synchronised across platforms. I deliberately choose to take the longer route, when it comes to these things, because I feel there is merit in the process of iteration and in the evaluation of one’s thoughts as they transform from formative to more descriptive and at last, final shapes. So I suppose my aversion to integrating a free-thought tool with a production tool (and a production tool from editing and review tools, and those from finishing tools) is the salt that should be taken with this discourse. 
Maybe I’m missing out, making life difficult for myself, but it seems to work for me.
Any future plans for more integration of the two programs?
That would be for Keith to say, but something that limits what can be done is that the two programs don’t share the same principle for how information should be organised. Scrivener is highly dependent upon depth. Nothing can exist without a parent, and everything can only have one parent. Things go in things, and how things are sorted into things is crucial to how they will be expressed in written form. The concept of hiding information and only letting the peaks show is important to the goal of the software: to make sense of massive amounts of text. Scapple on the other hand is flat, flat, flat. Everything is exposed. Every idea unavoidably takes up space, and the more complex the idea the more space it consumes. Nothing can be hidden, and even the most collective metaphor in the program, the background shape, is nothing more than a rectangle that moves beneath other rectangles, perhaps moving them too, but only optionally.
When it comes to import/export, that’s one thing, because information can be coerced into different forms, but when it comes to actually integrating live objects together from such disparate systems, you have to choose your battles very carefully.
But maybe you have something less tightly knit in mind?