NSIS - error handling for "unable to write file"

Question

I'm writing an NSIS installer. I'm trying to figure out how to capture the error when the file is unwritable (eg, if C:\Program Files\ is unwritable because the user doesn't have permissions).

The error text shown can be changed with "FileErrorText", but I'd like to call a function to do some work instead of showing a dialog box.

.onInstFailed is as close as it gets, but its shortcoming is that it isn't called until after the "Abort" button is pressed.

Anyone know if it is it possible to call a function instead of showing the dialog box to the user?