This can be done with pgbouncer and you can suspend the database so pgbouncer "holds" connections. If the application has high enough timeouts, it will look like it's just taking longer than normal to connect.
That specific case and if you can afford PgBouncer, I agree with you.
On a wider scale, Haproxy adds value - auto tracking on sync/async replicas, replica lag windows, falling back on master if all replicas lagging/down, so on.
