On a phone and without a reference handy but search for Franz and "Protocol Extension". It's a very short paper, and basically proposed a mechanism similar to vtables, but w/an extra level of indirection to allow whole interfaces to be slotted in, and to have subsequent loaded implementations addressed propagated down the class hierarchy on load.

(I use a similar mechanism of propagating vtables updates down the class hierarchy, but without the extra indirection - at cost of more memory - in my prototype Ruby compiler)