Seek within Large Object, within PL/* function?

Hi,
    I'm new here, apologies if this is the wrong list...

    I'm playing around with storing custom preprocessed data structures
within Large Objects.  I'd like to be able to write a custom function
that will, within a query, let me select out particular bytestrings from
the middle of a Large Object (within C, I think 'lo_lseek' and 'lo_read'
would get me what I want).

    I'd like to do so from some PL/* embedded language, rather than a
compiled C module.  I can't find any documentation on doing this,
though.  Is it possible?  Thoughts?

Thanks,
Adam