Thanks for working on this.
What I reported in my original post corresponds to using OPL version 0.94 (a beta build containing the newer GSM version 0.38).
After reading your observations with the stable OPL version 0.93, I downgraded to 0.93 and now I obtain the same result as you, i.e., my OSSC now reports 1037i 31kHz/60Hz when using the “640x480i” GSM mode. Likewise I get 1125i 33kHz/60Hz for the 1080i mode. Clearly this is a more accurate result than what the beta OPL build produces, so it’s probably best to ignore the beta version and just make OSSC’s firmware compatible with the output signal from the stable OPL version 0.93.
I’m glad to hear that support for higher-than-480-line interlaced modes is planned. What is the planned output mode for a 960i input? Would the OSSC convert it to 640x480p or linedouble it to 1280x960p? Ideally both would be possible, but is one easier to implement?