I got a chance to test the BKM-227 analogue option input on the F250 and performance was indeed the same as when feeding 480i (and its hacks like "240p") via HDMI. It's nice to have however for direct S-video and composite inputs. I have a few clips below, though my camera failed massively to pick up the awesome colors correctly.
This monitor is tragically close to being able to display a true "240p" 15khz signal. It shows a picture with "scanlines" (even on the OSD items like on a CRT BVM), but the picture is unstable and wobbles cyclically:
https://youtu.be/l2R1DpdlHVA (NESRGB)
https://youtu.be/VfilspbMdBA (PC-FX + OSD)
https://youtu.be/9ZCcacUAE34 (PC-FX)
Is it due to a sync issue? This does make me wonder what things would look like using the RGB card and different types of external sync, though I bet the results would be similar. It's sad because for an instant I can see a perfect 240p image, as crisp as on a CRT, albeit with inorganic-looking even scanlines.
The best solution for these older systems may be the most simple of line doublers, ideally with a scanline function, since 480p looks so good on this monitor. Anything that doesn't mess with the picture or colors would be ideal, since the monitor's colors and gamma can already look exactly like a BVM CRT (which is literally one of the gamma options paired with the SMPTE-C and EBU color space options). Maybe one of the older XRGB units? I suppose an OSSC would be fine, but those would most definitely alter the colors from what I've experienced.
480i as expected looks great (clip is X2 native scale, X1 looks even better but is tiny).
https://youtu.be/pd1to0yrtFo (Dreamcast)
I didn't capture anything using composite, but I did check it out and it looks, well, like composite on a CRT, which is impressive. There's quite a bit of shimmering with the default "3lines" comb filter, but the "3D" comb filter cleans it up very well without losing sharpness. I didn't look at much so I'm not sure in general which filter would be preferable.
Again I continue to be very impressed with this monitor for what it can do. If only it could have handled low res 15khz it would be almost a BVM CRT killer. On the other hand, while its blacks obviously beat the D24 (which is still pretty close in a pitch black room!), it doesn't match the peak brightness/highlights of the CRT unless I crank up the contrast to the point that the "over range" nag LED turns on. Could it be that this RGB OLED has less dynamic range than the CRT? As far as I know I'm not clipping whites on my CRT, though I did increase the sub contrast a bit to make it more punchy. Maybe this is the weakness of RGB OLED, which is why they're not being made anymore. Sony moved back to LCD (though a very nice one) for its current line of BVMs (HX-310). HDR killed RGB OLED, and in a few years it'll be even more of a dead tech than CRT. Hopefully we can again pick up the scraps for the next few years.