truncate capability in 8.1

Hi-
Using postgresql 8.1 on RHEL 5.3, would like to be able to update (e.g.,
overwrite) a large object in place. Ultimately, I'd like to delete the
original large object, add a new one, but keep the original Oid. I've
looked at using lo_open, lo_read, lo_write, but if the new file is smaller
than the original file, I need to truncate the original. Unfortunately,
lo_truncate isn't implemented until 8.3 so I was hoping to get some ideas
from other folks on the best way to do this using capability available in
8.1. Are there any "gotchas" directly manipulating the pglargeobject table?

- larry