For a STM32, can I use a quad SPI and SD card as a framebuffer for a LTDC LCD driver?
The SD card is 20 MHz and I know the performance would suck for an 800 x 480 display, but will the hardware do it?


No. The LCD driver needs to be able to access its framebuffer as memory. SDIO isn't memory-mapped, and QSPI won't drive an SD card.