simple graphics chips of the same type used in laptop docks?

Actually, there are motherboards that have up to 6 or 7 PCIE slots. Using multi-head graphics cards that gives you a lot of seats (and you will even be able to run OpenGL applications on all of them).

(Those who have been at FOSDEM in recent years may have seen/used such a multiseat system with 3 dual-head video cards, providing 6 seats running Compiz—and I have seen people play a first-person shooter on that machine!).