Joe_F, I am familiar with the concept of solfa, but only in the major key and I don't know how to deal with accidentals. But I'm not trained to sing in it, and I couldn't decipher a tune written out in it. In order to make sense of a tune written in solfa I would have to convert it to notation (although I don't read music well), and I would probably use ABC to do this.

ABC has become the default format for folk tunes. There are many collections of tunes on the internet, and it is used by the Traditional Tune Archive, the Morris Ring, the Village Music Project, and many others. There are plenty of online resources and free apps which will convert ABC into notation and play it back as MIDI, and it can also be imported into many score writing programmes. If you don't like the way a particular app or site deals with it there are probably others which will suit your needs.

Apart from its own merits as a format for music on the internet, for the sake of consistency with other folk music resources then ABC seems to me to be the way for Mudcat to go.