No proven solutions. But heres some ideas:
The kernel framebuffer draws the console if there is no X11. Perhaps you can exclude the second display from you X config?
I think the way to go (think because I havent done this myself, I like the full screen term) is to configure your external monitor as a separate framebuffer. Now you'll have one fb for the console and one for X11.
I have found several con2fb, which maps a tty to a framebuffer, something like "con2fb /dev/fb2 /dev/tty5" but can not find where to download it. Another fan had a similar search and the source is posted here.
Im thinking you could then take turns sending a tty to /dev/fb2 or extending your desktop... maybe xrandr?
Another way to try could be a multiseat set up. This typically has separate multiple kb and mouse for each display, but you could use Synergy2, and external kvm switch, custom config with hotkeys....? xorg xfree