BSP_DISCO_F746NG/Drivers/BSP/STM32746G-Discovery/stm32746g_discovery_camera.c is missing the ISR definitions:
void BSP_CAMERA_IRQHandler(void)
void BSP_CAMERA_DMA_IRQHandler(void)
For this reason, HAL_DCMI_IRQHandler and HAL_DMA_IRQHandler are never being called, meaning the consumer of the camera API never sees line event / frame event callbacks.
Adding these ISR definitions fixes the issue, and means I can run a camera demo which calls:
BSP_CAMERA_Init(RESOLUTION_R480x272)
BSP_CAMERA_ContinuousStart(buffer);
and grabs lines in
BSP_CAMERA_LineEventCallback().