Reason they used an external OSD driver is because of no input sync. The MCU can use the horizontal and vertical free running oscillators of the jungle chip internally, with a hardware counter counting lines since a vertical sync pulse, to get a set of registers that can be programmed to have a constant position on the screen, nearly irrespective of any input signal, just using the natural free running of the 2 oscillators. Then use a bit clock generated internally to put out the signal to override the actual jungle chip output, and pull the drive to full brightness as desired to generate the OSD.
This then will be nearly constant in position, and the jungle chip itself does not provide this, as it relies on having a valid vertical and horizontal sync to do the internal OSD, and otherwise will have a lot of jitter. Doing it externally makes it simpler, allowing you to use region specific chipsets, though I will guess that, if you remove that power filter board, there will be a set of traces for the SCART socket there, and probably the components for the socket as well, and this will allow you to select the SCART by simply forcing the SCART select input high with 5V, switching to the SCART instead of the line input. Right position for the SCART socket, and that blanking plate is a separate piece of the moulding, for different models.