It is certainly possible to set up Hoblink X11 to make applications in a Linux guest appear in seamless mode, just as one would display applications running on a remote system using X11. You could eliminate the whole X server in the VBox guest, and probably speed it up in the process. I can't think of any advantage to using the X server in the VBox guest.
There are some useful articles about Hoblink on os2voice detailing how to set it up. Using SSH makes it particularly easy, but if I were to do it I'd avoid it and use telnet, rexec, or xdm, because encryption eats cpu cycles, and it's pointless in a connection to a guest on the local system.
I used to do everything this way, with multiple physical systems on a single screen. I even had KDE on my OS/2 desktop, which, to say the least, is just weird.