Thanks for your comments mths.
I think it would probably make sense for mIRC to hide ISO channels in /list. It may even make sense to /part an auto-joined ISO channel and automatically /join the UTF-8 version. That would migrate many users to UTF-8 automatically.
As you know, my main hope is that this change will lead to a gradual change to UTF-8. I realize it will be frustrating initially but there does not seem to be another option - codepages will continue to be used, with all of their limitations and issues, without this change.
Regarding administrators: the only way to resolve this would be to add a UTF-8 enable/disable option. It is not quite that simple however - UTF-8 handling is embedded in a number of routines, such as file input/output, so it would have to be disabled in many places.