That's way more complex then it sounds. The joiner is kind of like a batch processor where we basically open each file one by one and then stitch them together at the edges. For a preview to work all of the files in the joiner would need to be opened concurrently and then we'd have to seamlessly stitch them together in real time. During output things can pause for a second to be reconfigured between files. Doing that same reconfiguring in real time so you could play it seamlessly may not even be possible without a pause or hiccup where the transition happens. And even if it works the memory requirements of opening all of the videos concurrently would be huge and probably not possible without upgrading the product to 64bit.*
* The main reason we we can't upgrade VRD to 64bit is because we depend on some 3rd party libs that aren't available as 64bit, and when you convert to 64bit everything has to be 64bit. We're working on ways to either get away from those libs or break them out into external executables, but that's a big project.