You can use Staz's add-on module to communicate which records are busy so 
that you can indeed lock part of the file.

I make it much simpler by not leaving the file open and using flushevents to 
make sure it is not left open. I open the file, get what I want, and shut it. 
If I am going to change something, I keep a note of which record then open 
the file again "RECORD" to that spot save the new record and close. I use "is 
filebusy" function from program generator to check before I open the file if 
busy, I just animate the cursor a few times and check again.