All right, success! I have a properly-looping OPUS file! The loopstart and looplength values given above do, in fact, work, with a tweaked plugin. Just trying to decide on the best way to package that for consumer use. I could do what I do for the tempo ratio and calculate fractions or let you input the sample rate for tracks that don't have one... or just let you do the math yourself and input decimal values.
A solution like this could repair the "Starting point" issue too.
EDIT: I'm leaning toward the sample rate option. Just have it ignored when the file actually has a proper sample rate.