I have decided to stop developing against the STM32F407 and switch to the STM32F413, the main reason being that this part has much more on-chip SRAM- 320KB to be exact. This will be enough for a 360 * 288 double frame buffer, which will give the same resolution as the MAX7456 chip used in the MinimOSD board.
Although this is only half the resolution of an analog TV picture, it is adequate for graphics and text overlays.
I have purchased a Nucleo-F413zh board and ported the code over to it, it is working but I have not taken advantage of the extra ram to create a larger frame buffer yet. That will be the next objective.
Weka OSD 