Joe asked about 'the easiest way to post'. But I think there are three questions:

1. Should the tune be stored in Mudcat itself - which means text. Or can it be a link to an outside server e.g, that Joe controls? That's the most important question IMO.
2. What's the easiest way to post?
   • Who? Joe or a member without privileges?
   • What might they start from? Sheet music? A youtube video?
3. What's easiest - or even usable - for the reader?
• Must it be usable by folk with little musical background?
• Should it be usable on mobile browsers - e.g. iPads
• Must they be able to hear it played?
• Could a javascript web player - e.g. for solfa - be but on the mudcat server so it could be played without external software?

ABC wins IMO if you want the tune to be within Mudcat - i.e. text. If it can be on a separate server then probably mp3 is easiest.

solfa is an interesting suggestion - especially if the poster can just write it with the tune in their head - tricky for complicated tunes I would have thought. Personally I'd find a score easier.

Anything that doesn't work on an iPad is useless IMO. (I don't use one, but a lot of musicians do and it's the most restrictive device.)