What is correct etiquette when modifying a library rountine?

I have modified the LCDDisplay library routine, originally written by James Lynch and previously modified by Peter Davenport, to add additional functionality. I have commented the changes with my name and date. There are no changes to the original functionality. As long as I keep this to myself I don't suppose there are any considerations. Should I keep it to myself or make it available? At this point, it makes a difference in the amount of documentation I include, whether it is just for me or for general use.

I think that the first thing to do is to describe the changes/enhancements you made. Then, if someone says “Hey, I could use that!”, you’d know what to do.

You could contact the original "maintainers" to bring your changes under their attention. There are several libs that have a new version every once and a while...