can't edit NSTextField in sheet at runtime

It turns out that I haphazardly found the solution, which I will now post for posterity...

The view that gets used as the sheet (NSWindow or NSPanel) needs to have a title bar. As soon as I toggled this in Interface Builder's inspector (Window → Appearance → Title Bar checkbox), I recompiled and the NSTextFields highlighted, tabbed, and accepted input like they would in any other view. Not sure why the title bar makes a difference, but there you have it.

The view does not absolutely have to have a title bar.

See Why NSWindow without styleMask:NSTitledWindowMask can not be keyWindow?

Which states: If you want a titleless window to be able to become a key window, you need to create a subclass of NSWindow and override -canBecomeKeyWindow as follows:

- (BOOL)canBecomeKeyWindow {
    return YES;
}

This worked for me.