Database with "override" tables

Hello,

I am in a situation where I have various tables (including data such as
a product list) that are read-only to me. I wish to provide the
functionality of changing this table: Removing items, modifying items,
creating new ones. My original idea is to use a second table that is
formatted similarly to the first, read-only table.

However, I can't just duplicate all the data and work in a separate
table, as the original table may be modified by an outside source at any
time. Generally, I prefer to use the updated data from the read-only
table, unless there has been reason to modify it in the past -- in which
case the update may be safely ignored, and I would continue to use the
second table.

The most effective set up I have come up with thus far is as follows:

CREATE TABLE initial_table (initial_id SERIAL PRIMARY KEY, desc TEXT);
CREATE TABLE override_table (override_id SERIAL PRIMARY KEY, initial_id
INT, desc TEXT);

An entry in override_table with an initial_id matching an initial_id in
initial_table would take precedence over the entry in initial_table. A
fairly simple SELECT statement can return the data I require:

SELECT COALESCE(ot.desc, it.desc) FROM initial_table it FULL JOIN
override_table ot ON it.initial_id = ot.initial_id;

So far so good, but then when I need to reference data in those two
tables from somewhere else, I need to reference both initial_id and
override_id on the result set from that table. I also end up using
COALESCE an uncomfortable amount (though, if this is required I will do
so). I would prefer to treat the initial + override tables as a single
SELECT set, but cannot come up with a way to do this cleanly, especially
with regards to the IDs.

I am willing to abandon this format of "overriding" the initial table if
it is way off-course, provided that the initial data be considered
read-only and update-able in the absence of "override" data.

All suggestions are greatly appreciated!

Thanks in advance,
Mike.