In case anyone else has experienced this problem as well, here's an update of what I've learned since posting the question.

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I spoke with H. Merijn Brand (author of DBD::Unify) at the July Amsterdam.pm meeting to see if he had any insights. According to him (and if this is wrong I probably am remembering it incorrectly), large object handling probably isn't implemented in the Postgres DBD because DBI currently lacks the specifications for doing so, which is the reason he did not implement large objects in his own driver. According to the DBI FAQ section 5.2, the answer to 'How do I handle BLOB data with DBI' is:

If the BLOB can fit into your memory then setting the LongReadLen attribute to a large enough value should be sufficient.

If not, ... To be written.

Postgres of course doesn't implement LongReadLen because of how it handles large objects. According to the DBI book ('Programming the Perl DBI' by Alligator Descartes and Tim Bunce, O'Reilly, ISBN 1-56592-699-4), the only method implemented by DBD::Pg is blob_read(), which is 'undocumented' (DBD::Pg has the following to say about it: 'Supported by this driver as proposed by DBI. Implemented by DBI but not documented, so this method might change.').

Based upon all this, I decided to use DBD::Pg's driver specific functions (lo_creat, lo_open, lo_import, etc) until something else is available.