Funny, I was thinking about the exact same thing a short time ago. Conceptually you'd need to fetch a byte or range of bytes (based on how much internal RAM you can spare) from the external RAM and then send those to the SSD1306. You'd need to modify the library so that writes to the frame buffer go to the external RAM instead of internal RAM. A lot of those writes involve array-style addressing and that obviously won't work with external RAM so you'd have some development work to do. You'll also need to modify reads from the frame buffer in the same way. Finally, you'll need to modify the code that updates the display from the frame buffer so that it fetches data from external RAM and sends it to the display. Of course this won't be as fast as using internal RAM. You could probably improve the performance with an interrupt driven routine for getting data from external RAM via SPI and then sending it to the display via I2C.
I was also thinking that this data transfer might perform reasonably well if it could be done using DMA but that's outside the realm of Arduino at that point.