with HDMI, only passthrough and Line2x works, but it works for all resolutions the Saturn and Megadrive can output.
This includes all resolution variants each console can output (4 of them for the Megadrive, and 28 for the Saturn, yes, 28). I’ve tested them all in PAL consoles, NTSC consoles, and PAL consoles 60hz modded. They all work fine. The only caveat is that you need to set the H-PLL pre/post values to 4, otherwise the Sonic 2 splitscreen mode on the Megadrive cannot sync properly.
Funnily enough, the progressive modes for the Saturn also get displayed properly.
Line3x works, but it is not properly centered. The image is too far on the bottom, a little part of the screen is cut off.
Line3x in PAL mode does not work.
Line4x and 5x do not work.
Everything is basically set to default mode.
I also tested with a basic HDMI to VGA adapter. You need to fiddle a bit in the TVs menu to get a centered image (under Setup -> PC setup).
From VGA input, Line2x, 3x and 4x will work, 5x will not (no matter whether I set 1920×1080, 1920×1200, or 1600×1200 – the TV manual states that these modes won’t work anyway). Passthrough mode will not work, I suppose the VGA input only accepts 31KHz signal. Also, PAL mode will not work at all.
VGA mode can take a little fiddling to properly calibrate, I recommend picking 858×480 (for Line2X) and 1280×720 (for Line3x) in the TVs menu. Line4x will default to SXGA with no selections offered. The problem with VGA is the aspect ratio and the refresh rate.
Line2x has proper aspect.
Line3x will, depending on input resolution, need switching between 4:3 and 16:9 on the TV (352x modes on the Saturn will end up widescreen, so you need to correct it down to 4:3; 320x modes will be instead too thin and need to be switched to 16:9).
Line4x has correct width in all modes, but the height is too low, so everything looks squashed. Not much, only a little, but this is noticeable after using Line2x mode.
I only tested the Saturn with VGA.
There is one more problem when using VGA. The console I tested outputs ~59.5Hz vertical sync, and while the TV correctly displays this value under the PC settings, it may not match it correctly. You get a small 1-frame skip every 1 second. This is present in all VGA modes, but NOT when using HDMI. Can be very annoying in scrolling backgrounds, but it also screws up interlaced modes (field order for the deinterlacing will end up switching).
I don’t know if this is caused by the HDMI->VGA converter or the TV itself, however. I’m using a noname cheap converter. Anyone knows if this can be fixed with a better converter? Using VGA would be better choice because it resyncs on resolution change twice as fast, and has less scaling artifacts on meshes in 352 px wide modes (this latter can be corrected with a custom sync mode – horizontal samplerate around ~455 iirc – but then you need to manually switch this around).
edit: when using a NTSC Saturn with the VGA converter, OSSC reports 59.79Hz, the TV reports 60.00Hz, and those 1-frame skips occur far less, if ever.