Pavane:
I believe I may have run into something like you are wanting a few weeks ago, but unfortunately I apparently didn't make very good notes. About all I can recall is that I believe I was looking for Celtic/Irish fiddle tunes, and hit on a fairly large collection.

There were some notes indicating "classification" codes for some of the tunes I looked at, and references to "other tunes with same/similar codes; but I didn't dig into the site enough to figure out exactly what he was doing.

Depending on how much detail you are looking for, converting everything to abc, and either transposing to a common key or using "semitones up/down/same" would give a good graph of a tune. If I infer correctly what you are looking for, the simple up/down/same notation would not give very good discrimination. You probably need up-how-far/down-how-far/same to be able to get much good out of it.

One of the things I was looking at at the time was variant versions of specific tunes. My observation would be that the "frills" would blow almost any system that tried to track all of the notes. There is also the problem of "pickup" notes, that could introduce an artificial "offset" in a simple ID code. Perhaps a coding based just on the "beat" notes would allow sorting down to manageable groups of tunes.
I have found the same tune written in Common, CutCommon, 4/4, 2/4, and 6/8 time signatures, and they all sound the same when played.

I will try to dig a little deeper in my notes when time permits, and get back to you if I find the site I think I remember. Unfortunately a quick look at recent notes indicates I may not have left myself a trail back.

John