I haven't seen it done with retro hardware.
On the other hand, some people have the "DIY ViDock"/eGPU approach for modern PCI-Express graphics cards on laptops with ExpressCard slots. People have even managed to get such setups to work with hacked NVIDIA Optimus drivers so they can use the internal display, but it does come at a slight performance penalty compared to just using an external monitor, on top of the usual performance penalties stemming from ExpressCard only supplying PCIe x1 bandwidth.
I'm sure it's possible with older cards like the PowerVR line, but don't be surprised if you run into issues like having to shutdown before connecting or disconnecting the card.
(Also, I'd have to admit that I'm more interested in using PCI or PCI-Express SOUND cards on a laptop, since only those have proper hardware sound acceleration for older games, and the software OpenAL device that Creative uses with their USB devices has its quirks. Meanwhile, you can find plenty of laptops with decent GPUs built-in...)