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.

Also check out this doc, especially 14.9 and this multihead howto that both discribe con2fb.

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

Good luck!