[exim] smarthost deferral?

Situation:
Network with multiple internet gateways.
Smarthost only accessible through one gateway.
The local mail host can send to the outside just fine -- the smarthost
is primarily used to avoid spooling locally.

Problem:
The line to the smarthost isn't 100% reliable (after AT&T took over the
phone company and lines).
Is there a way to defer a smarthost if it's not available, and continue
with normal routers? Something like:

smart_route
driver = manualroute
transport = remote_smtp
route_list = !+relay_to_domains smarthost:defer

dnslookup:
...

In other words, have the messages sent directly if the smarthost can't
be reached, instead of being queued for delivery to the smarthost? You
can specify more than one smarthost which will be tried in turn, but as
long as there's at least one, there's no fallthrough to the next router,
as far as I can tell?

Alternatively, could the route be regenerated (with smarthost removed)
at the transport side if timing out?

Regards,
--
*Art