Currently, when I want to query all "userland" tables, I write something like:
select ...
from pg_class cl, pg_namespace nsp
where cl.relnamespace = nsp.oid
and nsp.nspname not like 'pg_%
and nsp.nspname != 'information_schema';
A new boolean column named "indissystem" that's true only for system relations would make many maintenance queries cleaner, since they'd look like:
select ...
from pg_class cl, pg_namespace nsp
where cl.relnamespace = nsp.oid
and nsp.indissystem = false;
-- Death to <Redacted>, and butter sauce.
Don't boil me, I'm still alive.