It seems possible to edit IR codes once you have learned them from the original remote.
yes
What's the best utility for doing this (I keep seeing references to PENG but cannot see a download link on this site)
go to the file section under Pronto, and IR utilities, there are a few, but read the document on Hex first
What can you do / cannot do with these utilities
1) create codes that your device does not already know
2) the utilities only work on some codes/ manufacturers
Whats the basic procedure? Do you need to know hex programming (I don't!)
no programming, hex is a way of representing numbers, the Pronto uses hex to represent the on/off time of the IR