Grabber hardware and cheap adapters are generally very picky regarding resolutions. So you are usually better trying to route the signal through some good quality scaler. So the catch is, they're expensive.

I have an Extron DSC 301 HD, which has a HDMI-In at least, and can provide a wide range of output signals, basically anything a HDMI recorder might expect. I think this would be a good option to record video from MorphOS computers, although I haven't tested this yet with my Extron.

A second hand DSC 301 HD goes between 100 and 250 EUR on eBay at this point. Note that the Extron Firmware updates are paid though, but for private eBay users they're usually quite friendly, we got access to some updates from them after explaining that we use their equipment we bought second hand on non-profit events... (Yes, demoparties. :) )

I actually wanted to test MorphOS recording via this thing for a long time now. Gimme a few days, i'll try to upload something to YouTube.