If I understand your proposal correctly, it would be found out almost immediately, as file checksums would be altered and quite a few places do check them.
My idea was that, if you have an object file open with write permissions, you can assume that it has been created/modified anyway, and people will expect the checksum to change.
Of course this does not work if the checksum is calculated before the file is closed by your backdoored compiler.
Of course this does not work if the checksum is calculated before the file is closed by your backdoored compiler.