I can tell you that for NCSFs, other than the list of predefined variables from the archived wiki entry, I also used origFilename and origSDAT for tags. Those were tags I decided to use to help with conversions to NCSF and also as a way to document the source of the sequence in case anyone wanted to know.
I've also seen both track and disc used on a large amount of *sf files. track moreso, disc isn't really supported by all the *sf players out there.
Also, one that I have seen in a lot of *sf files are tags for ReplayGain, usually:
Not all the *sf players out there use those, but I know that my in_ncsf does, I believe even Neill's PSF plugin for Winamp does too (I can't recall its name offhand, just that it started with Highly).
One other one that I have also seen a lot but isn't listed on the wiki page is tagger. I believe the distinction between the *sfby and tagger fields is that *sfby is the one who created the set and tagger is the one who added the tags.
Most of the *_gain tags are treated as strings and are in the format of:
[+|-]X.XXdB
While most of the *_peak tags are possibly treated like floating point numbers and usually a decimal value with 9 digits after the decimal point, no truncating of 0s from the end, and only having a negative symbol but not a positive symbol, like:
1.123456789 1.000000000
I know that when Winamp, for example, writes the tag data to the files for ReplayGain, it writes them both as strings, thus why even a peak of 1.0 gets 9 zeros instead of 1 zero.