It sounds like MorphOS has yet to implement a means of communication across the A-Box border. After all, why else would their user space apps run in A-Box?
IIRC Hyperion has already said AOS4 doesn't have a sandbox for legacy. They defend this position by saying nearly all software will be legacy to begin with, but I don't fancy this trade-off. I remember all too well the hell of 68k compatibility in MacOS versus the joy of Classic (which is a virtual machine). With the former solution 68k and PPC apps could use the same libraries, there is less overhead, and it was easier to implement but old apps can still crash your system. With the latter solution all legacy apps run on a virtual machine running a complete OS then communicate to new apps (clipboard, drag+drop, IPC, networking, etc.) by patching the legacy OS. The disadvantages are the opposite of the former advantages.
I much prefer the sandbox solution even if it takes longer to implement.