I installed OPL 0.93 which didn’t contain that commit yet, but it has “640x480i” which apparently is the same mode but just misnamed. Activating it, OSSC reports 1037 lines interlaced @ 31.61kHz/60.97Hz which sounds valid for 960i mode. OSSC isn’t yet compatible with interlaced content that exceeds 480i/576i, but once support for that is added/fixed, 960i should work as well.
No idea why your setup generates invalid readings, maybe some PS2 models/bios do not support this mode. Did you actually get it to run correctly with some other video processor / display? Also, what OSSC reports if you select HDTV 1080i mode on your PS2?