I haven’t tried 720p50 as the RiscPC can generate 720p60 but it’s too low a resolution for a practical solution and the wrong aspect ratio for me at the moment anyway as I’m currently using a 16:10 monitor.
If I select 1080p50, it’s correctly recognised as 1125p @ 50hz by the OSSC LCD display but the advanced timing adjustments max out without being able to set them correctly although changing them does make a visible difference so I am adjusting the correct mode.
e.g. the H samplerate defaults to 2200 which is correct for 1080p60 but it should be 2640 for 1080p50 due to the large front porch and the adjustment maxes out at 2300.
So I guess I’m looking for an increase of the range of adjustment of the H samplerate so that the 1080 50hz modes can be brought into alignment (or as you mention a code change to auto switch 50/60) and the addition of the rest of the above VESA timings (or at least 1680×1050 if there are memory space issues) to the timing table which appears to be in video_modes.h