[eluser]Natebot[/eluser]
I would prefer a soft error with some attempt at error correcting. It could look in a conventional location first before complaining. The index.php file already does this for the other path constants, that's why I suggest SHAREPATH can be treated there too.
You could replicate that treatment inside MY_Loader too, for those who install it without altering their index.php, followed by a warning in the log.
Could we use the constants.php config file to store this path? I'm not sure what what point it is loaded. Would that be considered a bit more friendly than a new index.php?