I did manage to get it working eventually, but discovered that what I though was an RGB converter was not, in fact, putting out RGB. I ended up purchasing another converter, this time for composite video (HDMI to CVBS) and using one of the BNC video ports. This worked fine with the appropriate video settings (hdmi_group and hdmi_mode) in added in config.txt. The adapter does not work with 576i, but does recognize 576p. Possibly because this is a video monitor, the image does not fill the display like on a TV - there is a black border - but it does work.