Client-side compression

Not sure if this belongs here or on the admin or performance list.
Apologies if so. (And this may be a second posting as the first was from
an un-registered account. Further apologies)

My assumption is that any de/compression done by postgres would be
server-side.

We're considering minimizing bandwidth utilization by using client-side
compression on a column value that will typically be multi-megabyte in
size. We would use ALTER TABLE SET STORAGE EXTERNAL to prevent the
server from un-necessary compression.

Is this generally worthwhile? I haven't found any thread on the subject
of client-side compress so any pointer more than welcome.

Is there a great penalty for a query which delves into the value, given
that the server will not be aware it's compressed? I assume we're
pretty much on our own to prevent such actions (i.e. the app can never
query against this column via sql).