Well, a binary format or WAV is no biggie. I think dnordenberg is right though, I don’t think WAV will go down to less than 1kS/s. I know my preferred editor, GoldWave, won’t create a file lower than 1kS/s.
After all, it’s just a case of writing a program on the PC to translate the values into something we can work with - so whatever is easiest for the scope (i.e. uses less processing power, maybe saves bandwidth or memory on the SD card if that’s an issue) is probably gonna yield better sampling rates. After all, the logging “feature” will probably mainly be used to provide data for other applications, so it doesn’t really matter the format as long as it’s simple, documented and doesn’t chew more space than absolutely necessary.
EDIT: I would cast my vote for this feature. If anything, it opens a whole new world of possibilities for display and analysis on a more powerful platform - i.e. the PC.