I have finished porting the code over to the new STM32F413 Nucleo board and now have a frame buffer big enough to cover the whole screen. You can see the output here.
Word from Analog Devices regarding the chroma transient issue I mentioned earlier, is that this is a result of conversion to a YCrCb 4:2:2 digital format by the decoder. This format compresses the video data significantly, but at the expense of chroma information. Chroma is sacrificed because the human eye is much more sensitive to luma detail than chroma, so the loss is barely noticeable with a scene from ordinary television. However with coloured text and graphics it does become noticeable. From what I have observed, the effect is most noticeable with primary colours (pure red, blue or green). White or black pixels are the least affected and intermediate colours (yellow, magenta), while still affected, look better than primary colours. This makes sense when you consider that white pixels are almost entirely luma in the data stream, while primary colours contain the greatest proportion of chroma information.
For the time being I will have to live with this limitation. I will try to mitigate it by outlining text with black pixels (as most OSDs already do) and making coloured output at least 4 pixels wide (which will be 2 pixels in my half-resolution overlay).
It would make sense to render the altimeter, airspeed and AHI in white, and reserve colour for warning messages and status icons so that is what I am doing now. Ultimately this will be the sort of thing users can set according to their own preferences.
The way to avoid the chroma issue altogether would be to stream data as an RGB 4:4:4 stream which has no compression, or YCrCb 4:4:4. The ADV7341 encoder I am using understands RGB 4:4:4 but the ADV7184 cannot generate it and does not have a wide enough pixel bus.
Weka OSD 